A Study of Photoactive Materials for Solution-Processed

A Study of Photoactive Materials for Solution-Processed Thin-Film Solar Cells

by

Andrew Namespetra

Submitted in partial fulfilment of the requirements

for the degree of Master of Science

at

Dalhousie University

Halifax, Nova Scotia

August 2015

© Copyright by Andrew Namespetra, 2015

ii

DEDICATIONS

I dedicate this thesis to my family, for their love and support, and to all of my friends who

have accompanied me on the journey through grad school.

iii

TABLE OF CONTENTS

List of Tables ................................................................................................................ vi

List of Figures ................................................................................................................ vii

Abstract ......................................................................................................................... xi

List of Abbreviations Used ........................................................................................... xii

Acknowledgments ......................................................................................................... xiv

Chapter 1: Introduction ................................................................................................. 1

1.1 Background: Solar Photovoltaic Technologies ............................................. 1

1.2 Perovskite Solar Cells (PSCs) ....................................................................... 2

1.2.1 Perovskite Materials ......................................................................... 2

1.2.2 Photovoltaic Mechanism in Perovskite Solar Cells .......................... 3

1.2.3 Perovskite Solar Cell Device Construction ....................................... 5

1.3 Organic Solar Cells ........................................................................................ 6

1.3.1 Organic Semiconductor Materials...................................................... 6

1.3.2 Photovoltaic Mechanism in Organic Solar Cells .............................. 7

1.3.3 Organic Solar Cell Device Construction ........................................... 8

1.4 Solar Cell Device Testing and Performance Metrics .................................... 9

1.5 Thin Film Formation for Solar Cell Applications ......................................... 13

1.6 Project Goals ................................................................................................. 15

Chapter 2 – Characterization Techniques ..................................................................... 16

2.1 Optical Microscopy (OM) .............................................................................. 16

2.2 Scanning Electron Microscopy (SEM) .......................................................... 16

2.3 Atomic Force Microscopy (AFM) ................................................................. 16

2.4 X-ray Diffraction (XRD) ............................................................................... 19

2.5 UV-Visible (UV-Vis) Spectroscopy .............................................................. 20

2.6 Photoluminescence (PL) Spectroscopy ......................................................... 20

2.7 Cyclic Voltammetry (CV) ............................................................................. 21

Chapter 3: Thin Film Formation in Perovskite Solar Cells ........................................... 23

3.1 Background .................................................................................................... 23

3.1.1 Solution-Processed Perovskite Solar Cells ....................................... 23

3.1.2 Active Layer Components and Device Architecture ......................... 23

iv

3.1.3 Methods of Solution-Processed Perovskite Thin Films .................... 24

3.2 Experimental Methods – Sequential Dip Coating ......................................... 26

3.2.1 PbI2 and MAPbI3 Thin Films ............................................................. 26

3.2.2 Perovskite Solar Cell Fabrication Using Sequential Dip Coating .... 28

3.3 Perovskite Solar Cell Device Characterization ............................................. 29

3.4 Results and Discussion – Sequential Dip Coating ........................................ 30

3.4.1 Solution Concentration (Csolution) and Spin Speed .............................. 31

3.4.2 Substrate Temperature (Tsubstrate) ....................................................... 33

3.4.3 Device Performance Using Sequential Dip Coating ......................... 35

3.5 Experimental Methods – Sequential Spin Coating ........................................ 37

3.6 Results and Discussion – Sequential Spin Coating ....................................... 38

3.6.1 Annealing Conditions ....................................................................... 38

3.6.2 MAI Concentration ........................................................................... 40

3.6.3 Device Performance Using Sequential Spin Coating ....................... 42

3.7 Conclusions ................................................................................................... 43

Chapter 4: Organic Hole-Transporting Materials for Perovskite Solar Cells ............... 45

4.1 Background .................................................................................................... 45

4.2 Materials and Methods .................................................................................. 47

4.2.1 Materials ............................................................................................ 47

4.2.2 Thin Film Formation via Spin-Coating ............................................. 49

4.2.3 Perovskite Solar Cell Fabrication Procedure .................................... 50

4.3 Results and Discussion .................................................................................. 51

4.3.1 Materials Characterization ................................................................ 51

4.3.2 Perovskite Solar Cell Fabrication and Testing .................................. 55

4.3.3 Hole-Transporting Layer Surface Morphology ................................ 56

4.4 Conclusions ................................................................................................... 58

4.5 Future Work .................................................................................................... 60

Chapter 5: Star-Shaped Donor Materials for Small-Molecule Organic Solar Cells .... 62

5.1 Background .................................................................................................... 62

5.1.1. Transition from Hole-Transporting Materials to Donors ................. 62

5.1.2 Selection of a Complementary Acceptor .......................................... 62

v

5.1.3 Custom-Made Donor-Acceptor Pairs ................................................ 64

5.2 Materials and Methods .................................................................................. 65

5.2.1 Materials ............................................................................................ 65

5.2.2 Thin-Film Formation via Spin Coating ............................................. 67

5.2.3 Organic Solar Cell Fabrication Procedure ........................................ 68

5.2.4 Organic Solar Cell Testing ................................................................ 69

5.3 Results and Discussion, Part I ....................................................................... 69

5.3.1 Materials Characterization, Part I ..................................................... 69

5.3.2 Thin-Film Characterization, Part I .................................................... 72

5.3.3 Organic Solar Cell Testing, Part I ..................................................... 77

5.3.4 Conclusions, Part I ............................................................................ 79

5.4 Results and Discussion, Part II ...................................................................... 80

5.4.1 New Acceptors: A Molecular Design Strategy ................................. 80

5.4.2 Materials Characterization, Part II .................................................... 82

5.4.3 Thin-Film Characterization, Part II .................................................... 83

5.4.4 Organic Solar Cell Testing, Part II .................................................... 88

5.4.5 Conclusions, Part II ........................................................................... 88

5.5 Outlook and Future Work .............................................................................. 89

Chapter 6 – Conclusions ............................................................................................... 92

References ..................................................................................................................... 95

Appendix A – Solution Preparations ............................................................................ 104

Appendix B – Synthesis of CH3NH3I ............................................................................ 106

Appendix C – Other Factors Influencing Lead Iodide Film Morphology .................... 107

Appendix D – Perovskite X-ray Diffraction Peak Intensities ....................................... 109

Appendix E – Thiophene in Organic Small Molecules ................................................ 110

Appendix F – The Effect of Dopants on Hole-transport ............................................... 113

Appendix G – The Effect of Non-conjugated Polymer Additives on Film

Formation of Hole-transport Layers ............................................................................. 114

Appendix H – 3D Non-Fullerene Acceptors ................................................................. 117

Appendix I – Varying Donor-Acceptor Ratios ............................................................. 119

Appendix J – Solvent Additives and Solvent Vapour Annealing ................................. 121

vi

LIST OF TABLES

Table 3.1. Initial and final (i.e., after 30 s) temperatures of glass and FTO

substrates heated on a hotplate set to Thotplate. ....................................................... 33

Table 3.2. J-V metrics for PHJ PSCs, showing the effect of substrate

temperatures at a spin speed of 3000 rpm ........................................................... 36

Table 3.3. J-V metrics for PHJ PSCs, showing the effect of substrate

temperatures at a spin speed of 1000 rpm ........................................................... 37

Table 4.1. HTM identification .................................................................................... 49

Table 4.2. HTM deposition parameters ........................................................................ 50

Table 4.3. Summary of optical and electronic parameters of HTMs ............................ 55

Table 4.4. Summary of performance metrics for the champion cells with

different HTLs ..................................................................................................... 56

Table 4.5. Roughness parameters of HTL films on perovskite obtained from

2D AFM images .................................................................................................. 58

Table 5.1. Molecular components used in the acceptor derivatives ............................. 81

Table D1. Peak areas corresponding to XRD plots in Figure 3.15, determined by

applying the Pearson VII function and using Jade software ......................................... 109

Table G1. Summary of J-V characteristics of devices with HTM04 as the HTM

with and without non-conjugated polymer additives .......................................... 115

Table G2. Roughness parameters for films of HTM04 with and without

polymer additives on perovskite obtained from 2D AFM images ...................... 116

Table J1. Compositions of solvents with solvent-additive ........................................... 121

vii

LIST OF FIGURES

Figure 1.1. CH3NH3PbI3 crystal structure ............................................................... 3

Figure 1.2. Energy level diagram and device architecture for a PSC ..................... 5

Figure 1.3. Examples of π-conjugated organic materials for OSCs ....................... 7

Figure 1.4. Energy level diagram and device architecture for an OSC ................... 8

Figure 1.5. Bulk and planar heterojunction active layers for OSCs ....................... 9

Figure 1.6. Illustrated I-V curves for a SC .............................................................. 10

Figure 1.7. Circuit diagrams for SCs operating under dark and light conditions .... 12

Figure 1.8. Illustrated I-V curves showing the detrimental effects on FF with

increasing RS and decreasing RSH .................................................................. 13

Figure 1.9. An illustration of the stages involved in the spin-casting procedure .... 14

Figure 2.1. An illustration of the operation of an AFM .......................................... 17

Figure 3.1. Device architectures for planar and mesostructured PSCs ................... 24

Figure 3.2. Photographs depicting the stages of the SDC procedure ...................... 28

Figure 3.3. A photograph of a substrate with 16 completed PSC devices .............. 30

Figure 3.4. OM images of PbI2 films spin-cast on TiO2 compact films showing

the effect of solution concentration and spin speed ....................................... 32

Figure 3.5. SEM images of the CH3NH3PbI3 films after conversion of the PbI2

films via SDC, showing the effect of solution concentration and spin

speed .............................................................................................................. 32

Figure 3.6. SEM images of PbI2 films spin-cast on TiO2 compact films and the

corresponding CH3NH3PbI3 films after conversion via dipping, showing

the effect of spin speed and substrate temperature ......................................... 34

Figure 3.7. SEM images of PbI2 films spin-cast on TiO2 compact films and the


the effect of substrate temperature ................................................................. 35

Figure 3.8. J-V curves for the best-performing PHJ PSCs with PbI2 films

deposited at 3000 rpm on substrates heated to different temperatures .......... 36

Figure 3.9. J-V characteristics of the best-performing PHJ PSCs with PbI2 films

deposited at 1000 rpm on substrates heated to different temperatures .......... 37

Figure 3.10. Photographs of films depicting stages of SSC procedure .................. 37

viii

Figure 3.11. An XRD pattern of PbI2 on TiO2 compared to CH3NH3PbI3 films

after annealing at 100 °C for 1 h in ambient and inert conditions ................. 39

Figure 3.12. 2D AFM topographical maps of the surface of two perovskite films

– one annealed in ambient and one annealed in inert atmosphere ................. 40

Figure 3.13. XRD patterns of CH3NH3PbI3 thin film formed via the SSC

method with different concentrations of MAI precursor solution ................. 41

Figure 3.14. Relative peak intensity of the highest-intensity (001) PbI2 XRD

peak plotted as a function of MAI precursor concentration .......................... 42

Figure 3.15. J-V curves for the best-performing PSC devices fabricated via the

SSC method using an MAI concentration of 40 mg/mL and 50 mg/mL ...... 43

Figure 4.1. The design motif for HTMs with a TPA core and two examples of

TPA-based small-molecule HTMs ................................................................ 46

Figure 4.2. Molecular structures of the five HTMs investigated in this study ....... 49

Figure 4.3. Energy level diagram for all components in the PSCs ........................ 53

Figure 4.4. CVs depicting the onset of oxidation for each of the five HTMs ........ 53

Figure 4.5. UV-Vis absorption spectra of HTMs for: a) solutions with CHCl3

as the solvent and b) thin-films on glass substrates. ...................................... 54

Figure 4.6. J-V curves of the best-performing cells with different HTLs ............... 55

Figure 4.7. 2D AFM images of HTL surfaces of the five materials under study ... 58

Figure 4.8. Proposed modifications to HTM01 and HTM02 for future studies

in TPA-based HTMs ...................................................................................... 61

Figure 5.1. An illustration of the concept of opposing molecular topology in

donor-acceptor BHJ OPVs ............................................................................ 63

Figure 5.2. Structures of D1, D2 and A1 ................................................................ 67

Figure 5.3.a) Solution UV-Vis absorption spectra, b) CV plots, c) an energy-

level diagram for D1, D2 and A1 and d) a table summarizing the

optoelectronic properties of the three materials ............................................ 71

Figure 5.4. UV-Vis absorption spectra acquired for neat films of a) D1, b) D2

and c) A1 during the sequential thermal annealing experiment .................... 73

Figure 5.5. UV-Vis absorption spectra acquired for blend films of a) D1-A1

and b) D2-A1 during the sequential thermal annealing experiment .............. 73

ix

Figure 5.6. PL spectra comparing as-cast neat and blend films .............................. 75

Figure 5.7. XRD patterns one-component thin-films of a) D1 and b) A1 and c)

a two-component blend film acquired before and after thermal annealing

........................................................................................................................ 76

Figure 5.8. XRD patterns of separate films of D1-A1 blends (1:1 weight ratio)

comparing the effect on crystallinity of annealing temperature .................... 77

Figure 5.9. a) J-V curves of representative devices with D1-A1 active-layer

blends in a 1:1 ratio. b) An illustration of the layered “conventional”-

style architecture. c) A table of performance metrics. d) Photographs of

the completed devices .................................................................................. 78

Figure 5.10. AFM images of the D1:A1 active-layer surfaces of the devices ........ 79

Figure 5.11. Structures of A1 and the four new derivatives ................................... 81

Figure 5.12. a) Energy level diagram of the frontier molecular orbitals of D1

and the five acceptors determined by CV. b) CV plots for the acceptor

derivatives, normalized such that the current of the first oxidation peak

is set to unity .................................................................................................. 82

Figure 5.13. Thin-film characterization of D1:Ax blended films (1:1 by-weight)

as-cast (dashed lines) and after thermally annealing at 180 °C for 10 min

(solid lines). UV-Vis absorption spectra (a-e) and XRD patterns (g-k) are

shown in the left- and right-hand columns, respectively ............................... 85

Figure 5.14. 2D AFM topographical images of the surfaces of thin-film blends

of A1 with acceptors A2, A3, A4 and A5, as-cast and after annealing at

180 °C for 10 min .......................................................................................... 87

Figure 5.15. A comparison of the structures of A2 and AX ................................... 91

Figure C1. SEM images of PbI2 films spin-cast on TiO2 compact films and the


the effect of solution temperature .................................................................. 107

Figure C2. SEM images of PbI2 films spin-cast on TiO2 compact films and the


the effect of different atmospheres ................................................................ 108

x

Figure E1. Structures of small-molecule HTMs for PSCs with thiophene

functionalities ................................................................................................ 111

Figure E2. Structures of polymer HTMs for PSCs with thiophene

functionalities ................................................................................................ 112

Figure F1. A comparison of J-V curves of the best-performing devices with

Spiro-OMeTAD doped with Li-TFSI and tBP against devices undoped

Spiro-OMeTAD ............................................................................................. 113

Figure G1. A comparison J-V curves for the best-performing devices with

HTM04 with and without non-conjugated polymer additives ....................... 114

Figure G2. 2D AFM images of HTL surfaces of HTM04 with and without

polymer additives on a perovskite film ......................................................... 115

Figure H1. Examples of 3D non-fullerene acceptors .............................................. 118

Figure I1. UV-Vis absorption spectra acquired for as-cast blend films of a) D1-

A1 and b) D2-A1 with varying D:A weight ratios ........................................ 119

Figure I2. XRD patterns of thin-film blends with D1-A1 ratios of a) 1:1 and b)

1:2 acquired before and after thermal annealing ........................................... 120

Figure J1. UV-Vis spectra of D1-A1 blend thin films (1:1 weight ratio) with

various amounts of solvent additives ............................................................ 122

Figure J2. A photograph of a D1-A1 thin film undergoing SVA in an annealing

chamber ......................................................................................................... 123

Figure J3. a) UV-Vis spectra of a D1-A1 blend thin film (1:1 weight ratio)

before and after sequential SVA. b) XRD patterns of a D1-A1 blend thin

film before and after SVA for 20 min ........................................................... 124

xi

ABSTRACT

The overall goal of the research projects herein is to fabricate low-cost and efficient

thin-film perovskite and organic solar cells. The fabrication of a planar-heterojunction

perovskite solar cell was first optimized by investigating factors that influence perovskite

thin film formation. The optimized sequential spin-coating method produces uniform

perovskite thin films and yields devices achieving power conversion efficiencies beyond

13 %. Secondly, cheap, organic hole-transporting materials based on a triphenylamine core

were investigated as alternatives to spiro-OMeTAD in perovskite solar cells, but poor

solubility led to deficiencies in photovoltaic performance. Finally, it was shown that both

solution processing techniques and molecular design can influence the morphology,

electronic properties and crystallinity of thin films consisting of a fullerene-free donor-

acceptor pair for applications in organic solar cells. It was determined that solution

processing and the molecular composition of photoactive materials influences thin film

properties and device performance in perovskite and organic solar cells.

xii

LIST OF ABBREVIATIONS USED

AFM Atomic force microscopy

BDT Benzodithiophene

BHJ Bulk-Heterojunction

CB Chlorobenzene

CBM Conduction band minimum

CV Cyclic voltammetry/voltammogram

D-A Donor-Acceptor

DFT Density functional theory

DI Deionized

DIO 1,8-diiodooctane

DMF Dimethylformamide

DPP Diketopyrrolopyrrole

EDOT 3,4-ethylenedioxythiophene

ETL Electron-transporting layer

ETM Electron-transporting material

Fc Ferrocene

FTO Fluorine-doped tin oxide

FWHM Full-width half-height maximum

HOMO Highest occupied molecular orbital

HTL Hole-transporting layer

HTM Hole-transporting material

ITO Indium-doped tin oxide

I-V Current-voltage

J-V Current density-voltage

Li-TFSI Lithium trifluoromethanesulfonate

LUMO Lowest unoccupied molecular orbital

MAI Methylammonium iodide

MAPbI3 Methylammonium lead iodide

1-NA 1-naphthaldehyde

NHE Normal hydrogen electrode

NMR Nuclear magnetic spectroscopy

OFET Organic field-effect transistor

OM Optical microscope/microscopy

OPV Organic photovoltaic

OSC Organic solar cell

PDMS Polydimethylsiloxane

P3HT Poly-3-hexylthiophene

PEDOT:PSS Ethylenedioxythiophene : poly(styrene sulphonate)

PHJ Planar heterojunction

PCBM Phenyl-C61-butyric acid methyl ester

PCE Power conversion efficiency

PDI Perylene diimide

PL Photoluminescence

PSC Perovskite solar cell

PTFE Polytetrafluoroethylene

xiii

PVDF Polyvinylidene fluoride

R2R Roll-to-roll

RMS Root mean square

RT Room temperature

SDC Sequential dip-coating

SEM Scanning electron microscopy

SPM Scanning probe microscopy

SSC Sequential spin-coating

SVA Solvent vapour annealing

tBP tert-butylpyridine

4-TFBA 4-trifluoromethyl benzoic acid

TGA Thermogravimetric analysis

TFT Thin-film transistor

THF Tetrahydrofuran

TPA Triphenylamine

UPS Ultraviolet photoelectron spectroscopy

UV-Vis Ultraviolet-Visible

VBM Valence band maximum

XPS X-ray photoelectron spectroscopy

XRD X-ray diffraction

xiv

ACKNOWLEDGEMENTS

First and foremost, I would like to thank my supervisors Prof. Gregory Welch and Prof. Ian

Hill for their continued guidance and mentorship throughout my program. I also

thank Prof. Jean Burnell for his assistance on my thesis and Prof. Mary Anne White

and Prof. Jeffery Dahn for their roles as committee members.

This work was made possible due to collaborations with other members of the Welch and

Hill research groups. I would like to thank each contributing member individually:

i) Arthur Hendsbee for: i) synthesis of organic materials including D1 and A1-A5,

HTM01 and HTM02, ii) acquisition of CV data for D1, A1, A3, A4 and A5, iii)

XRD patterns of the thermally-annealed blend films of D1:A1;

ii) Elizabeth Kitching for: i) spin-coating of the D1:A1 1:1 blends, ii) thermal-

annealing of the corresponding films, and iii) acquisition of UV-Vis spectra of

the films;

iii) Jetsuda Areephong for: i) synthesis of HTM03 and DPP1 and ii) acquisition of

the corresponding solution UV-Vis and CV data for each of these materials;

iv) Seth McAfee for running the CV experiments for HTM04;

v) Jess Topple for her guidance and advice on use of the AFM; and

vi) Charlotte Clegg for help and support with optimizations of perovskite thin-

films.

I wish to thank Dalhousie faculty members: Prof. Mark Obrovac for the use of his X-ray

diffractometer and Prof. Jeff Dahn for the use of his SEM.

I would also like to thank Dr. Nicholas Vukotic for helping me with the analysis of the

XRD powder patterns in Figure 3.15 using Jade software.

1

CHAPTER 1 - INTRODUCTION

1.1 Background: Solar Photovoltaic Technology

As the global demand for energy is projected to rise over the next few decades,1

clean, renewable sources of energy are becoming increasingly important. Energy-capture

systems such as wind, solar and hydroelectric are important to offset the dependence on

fossil fuels for energy conversion.

Solar energy is particularly promising given the vast resource provided by the Sun.

For example, 4.3×1020 J of energy from the Sun strikes the Earth every hour, which greatly

exceeds the amount of energy consumed in the United States in an entire year.2,3 Other

strategies exist to capture energy from the Sun (e.g., solar thermal, solar fuels, etc.), but

solar photovoltaic (PV) technology is the conversion of solar energy directly into

electricity.

There are three general types of photovoltaic technology: first-, second- and third-

generation. First-generation PV utilizes crystalline silicon as the active material; it is the

most widespread PV technology and currently dominates the market. Second-generation

thin-film PV technologies, including CIGS (copper indium gallium selenide), GaAs, CdTe,

can be made with lower quantities of materials, but typically consist of scarce and/or toxic

elements. Third-generation photovoltaic technologies utilize soluble photoactive materials.

Therefore, the active layers can be formed via common printing and coating techniques,4

offering the potential for rapid roll-to-roll (R2R) printing on an industrial scale.5 These

techniques can be performed at low temperatures, which opens the door for lightweight and

flexible substrates such as foils and plastics. In general, from first- to second- to third-

generation photovoltaics, the overall production cost can be reduced, but power conversion

2

efficiencies (PCEs) also decrease. It is therefore a challenge to increase the PCE of third-

generation PV to realize low-cost and efficient solar cells.

Two promising third-generation PV technologies are perovskite solar cells (PSCs)

and organic solar cells (OSCs). PSCs have rapidly gained attention in the scientific

community since their discovery in 2009.6 To date, the most efficient lab-scale perovskite

solar cells deliver a PCE greater than 19 %.7 Although these were small area (10 mm2)

devices, they have the potential to compete with multi-crystalline Si solar cells, which

demonstrate PCEs of ca. 21 % in larger-area modules.8

Organic solar cells employ organic π-conjugated polymers or small molecules as

light-harvesters. While less efficient than PSCs and other inorganic photovoltaic

technologies, OSCs have also seen great improvements in efficiency over the past three

decades. Since one of the first reports of a functional organic solar cell in 1986 by Tang,9

the efficiency of devices has improved from under 1% to the record of 11% in 2015.8

Despite similarities in operating principles, perovskite and organic solar cells have

significant differences. Therefore, a discussion of both systems in terms of materials and

device operation is warranted.

1.2. Perovskite Solar Cells (PSCs)

1.2.1. Perovskite Materials

The term “perovskite” refers to the class of crystal structures named after the

Russian mineralogist Perovski. In general, perovskites have an “ABX3” chemical formula,

where the A and B cations are 12- and 6-coordinate with respect to the X anion (X is a

halogen or oxygen). For years, inorganic-organic perovskites have remained interesting

materials in electronic applications for their abundance, conductivity and excitonic

properties.10,11 In these materials, the size of the “A” cation governs the dimensionality of

3

the structure. Small monovalent cations such as methylammonium (MA) and

formamidium form 3D structures,12 whereas large aryl groups can form the 2D layered

structure.13–15 The 3D structure is more relevant in photovoltaics for its lower bandgap and

lower exciton binding energy.16

In 2009, Kojima et al., demonstrated the successful fabrication of a solar cell using

methyl ammonium lead iodide (CH3NH3PbI3 or “MAPbI3”) as the light-harvesting

sensitizer (the crystal structure is shown in Figure 1.1).6 MAPbI3 continues to be used as

the standard active layer for PSCs, but improvements in device and material design (e.g.,

the replacement of electrolyte with solid-state organic hole-transporting layers (HTLs))17

have led to vast increases in performance and stability.

Figure 1.1. An image of the 110 projection of MAPbI3 simulated using Vesta software

from crystal structure information acquired by Stoumpos et al.18 The PbI6 octahedra are

darkened for visual contrast. The Pb, I, C and N atoms are represented by black, purple,

brown and silver icons, respectively.

1.2.2. Photovoltaic Mechanism in Perovskite Solar Cells

Photovoltaic performance in devices, whether in PSCs or OSCs, requires the

absorption of photons, which is dependent on both the i) bandgap and ii) extinction

coefficient of the active material. Materials for photovoltaic applications are generally

4

designed and/or selected to match the bandgap with the region of the solar spectrum that

displays the greatest flux, that is, the visible region which corresponds to wavelengths from

400 to 700 nm.

In general, a photovoltaic device operates by absorbing photons and generating

charge carriers. Photoexcitation and charge generation occurs in the active layer which

contains the photoactive material. The operation of a PSC is illustrated in Figure 1.2 a).

Since MAPbI3 has a high dielectric constant (ε ~ 18),19 photoexcitation in the perovskite

generates free charge carriers; the electrons and holes separate immediately.20 Electrons

are the mobile charges and “holes” refer to the vacancies left by the electrons. The free

electrons and holes move through conduction band minimum (CBM) and the valence band

maximum (VBM), respectively, within the perovskite thin film active layer. The electrons

diffuse to the interface of the electron-transporting (ETM), are transferred to the CBM of

the ETM and are then collected at the cathode. In the PSCs used in the studies herein, TiO2

is used as the ETM. Although TiO2 has a very wide bandgap (~ 3 eV)21 and may be viewed

as an insulator, the CBM of TiO2 aligns with the CBM of CH3NH3PbI3 and is therefore

accessible for electron transfer.22 On the opposite side of the active layer, holes diffuse to

the interface of the hole-transporting material (HTM) and are transferred to the highest

occupied molecular orbital (HOMO) of the HTM. In Figure 1.2 a), holes are depicted as

moving charges, however, a more accurate picture is movement of electrons from the anode

to fill the vacancies in the valence band maximum (VBM) of the perovskite left after

photoexcitation. Using ultraviolet photoelectron spectroscopy (UPS), the VBM for

MAPbI3 has been measured to be −5.43 eV versus energy of an electron in vacuum.17 It

was also possible to determine the CBM (−3.93 eV) by applying the optical bandgap.

5

Figure 1.2. a) An energy level diagram illustrating the mechanism of charge generation in

a PSC and b) a diagram depicting the device architecture of a PSC.

1.2.3. Device Construction

Thin-film perovskite solar cells s (and organic solar cells) are constructed in

layered, “sandwich”-style devices, with each layer contributing to the generation of current.

Complementary charge-transporting materials make contact with the active layer to extract

the charges and generate a current. Charge transport is facilitated by movement of electrons

from the higher-energy levels of the active material to the lower-energy levels of adjacent

materials along a gradient. For this reason, selection of materials with appropriate energy

levels is critical. Figure 1.2 b) depicts the typical device design or “architecture” for a

PSC. Alternative architectures referred to as “inverted” devices exist for PSCs23–26

however, these are not discussed herein. The devices are fabricated layer-by-layer on a

glass substrate coated with a transparent conductive oxide, such as fluorine-doped tin oxide

(FTO), that allows light to enter the device. The active layer is positioned in the middle of

the device, with charge-transport layers making physical contact above and below.

In “normal” PSC devices, FTO serves as the cathode, where electrons are collected.

The second layer is a dense thin film of TiO2 which is both a hole-blocking and electron-

transporting layer (ETL). The third layer is the perovskite active layer, which is the site of

6

photoexcitation. Covering the perovskite is the hole-transporting layer (HTL) which

consists of a wide bandgap organic semiconductor (e.g., 2,2’,7,7’-tetrakis(N,N-di-p-

methoxyphenylamine)-9,9’-spirobifluorene (spiro-OMeTAD), vide infra) that both

harvests free holes from the perovskite and blocks the flow of electrons in the wrong

direction (thus reducing the instances of recombination). Lastly, the top metal contact

(usually Ag or Au) acts as the anode, which “collects holes” or supplies electrons to system.

1.3. Organic Solar Cells

1.3.1. Organic Semiconductor Materials

Organic semiconductors have conjugated structures with alternating double-single

bonds and a continuous overlap of p-orbitals. This results in extensive π-electron

delocalization and a narrowing of the bandgap energy, which is the difference in energy of

the HOMO and the lowest-unoccupied molecular orbital (LUMO). These electronic

properties allow for the absorption of photons.

Organic semiconductors exist in two categories: donors and acceptors. In general,

acceptors have lower molecular orbital energy levels (relative to the ionization energy of

hydrogen in vacuum) and correspondingly higher electron-affinity. Donors have elevated

LUMO levels and therefore a lower electron affinity. The photoactive layer of an organic

solar cell consists of an electron donor and an electron acceptor in physical contact. Figure

1.3 depicts the structures of some common organic materials that function in organic solar

cellss. Some of the best-performing acceptors are fullerene derivatives, such as phenyl-

C61-butyric acid or “PCBM,” which will be discussed in greater detail in subsequent

chapters. Also shown are examples of a polymer (poly 3-hexylthiophene or “P3HT”) and

small-molecule (5,5’-bis[(4-(7-hexylthiophen-2-yl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-

7

c]pyridine]-3,3’-di-2-ethylhexylsilylene-2’2’-bithiophene or “DTS(PTTh2)2”)27 donor

material.

Figure 1.3. Examples of π-conjugated organic materials for OSCs.

1.3.2. Photovoltaic Mechanism in Organic Solar Cells

The photovoltaic mechanism in OSCs is illustrated in Figure 1.4 a). In contrast to

perovskite materials, organic semiconductors have lower dielectric constants (ε ~ 3).28

Consequently, light-absorption of the donor in an organic solar cell generates a bound

electron-hole pair known as an exciton. The exciton migrates to the D-A interface, where

it can then dissociate into free charge carriers.29 However, dissociation of the exciton is

met by a Coulombic barrier known as the binding energy. As shown by Hill et al.,30 the

binding energy of organic semiconductors can be quite large (i.e., 0.4 to 1.4 eV) which

necessitates a sufficient energy offset between the LUMO of the donor and the LUMO of

the acceptor to induce charge separation. After separation, the free electrons and holes are

transported through the acceptor and donor, respectively, and collected at the electrodes.

8

Figure 1.4. a) An energy level diagram illustrating the mechanism of charge generation in

an OSC and b) a diagram depicting the device architecture of an OSC.

1.3.3. Organic Solar Cell Device Architecture

In conventional organic solar cell devices (Figure 1.4 b)), indium-doped tin oxide

(ITO) serves as the anode (the site of hole-collection). The second layer is the hole-

transporting layer, which is a mixture of conductive polymers poly(3,4-

ethylenedioxythiophene) and polystyrene sulfonate in what is known as PEDOT:PSS. The

third layer is the active layer composed of an interface of the donor and acceptor. The two

materials come into contact at a D-A heterojunction, which is facilitated by the design of

the active-layer. It is important to note that OSCs can also be built with an inverted

architecture,31 however these designs were not implemented in the projects herein and are

therefore not discussed.

Two main types of active-layers in OSCs exist: a planar heterojunction (PHJ) and a

bulk heterojunction (BHJ) (depicted in Figure 1.5). Since the exciton diffusion length in

organic semiconductors is very short (10-20 nm), PHJ cells must have very thin donor and

acceptor films to allow the excitons to reach the D-A interface before they decay. In

contrast, the BHJ consists of an intimate mixture of the two materials, creating percolating

9

networks within the “bulk” of the active layer.32 A BHJ film is formed simply by dissolving

the two materials in a common solvent, casting the solution, and allowing the donor and

acceptor to self-assemble into distinct domains. Advantageously, BHJ active layers can be

made thicker (~100 nm) than a PHJ, allowing for a greater quantity of light-absorbing

material, and consequently higher current output. Lastly, the top contact is a low work-

function metal, such as Al as the cathode. Metals are typically deposited by thermal

evaporation under vacuum.

Figure 1.5. Cross-sectional illustrations of OSC devices, comparing planar and bulk

heterojunction active layers in a conventional-style architecture.

1.4. Solar Cell Device Testing and Performance Metrics

Testing the photovoltaic performance is critical for the assessment of a solar cell

and the quality of the functional materials (e.g., hole-transporting materials in a perovskite

solar cell or a D-A pair in an organic solar cell). Measurement of the current (I) produced

by the cell as a function of applied bias or voltage (V) generates an I-V curve (illustrated in

Figure 1.6).

10

Figure 1.6. An illustration of I-V curves for a SC tested in the dark and in the light (i.e.,

under illumination).

In many cases, current is reported per unit area of the cell as a current density (J).

By convention, the photogenerated current is negative on the I-V curve and the product of

I and V is the power generated by the device (P):

P = IV. (1)

From the I-V (or J-V) curve, many important parameters can be determined. First,

the open-circuit voltage or Voc, occurs when no current flows external to the cell. The

maximum Voc of a perovskite solar cell is the difference in potential energy between the

CBM of the ETL (e.g., TiO2) and the VBM of the HTL; in other words, the Voc is the

difference in potential energy of the excited electron and the collected hole.33 Comparably,

the Voc of an organic solar cell is limited by potential difference between the LUMO of the

acceptor and the HOMO of the donor. The second parameter is the short-circuit current

(Isc or Jsc) which occurs when the voltage equals zero and represents the maximum current

11

output of the SC. Multiplying I by V gives P as a function of V which gives the maximum

power (Pmax) generated by the cell. The current and voltage at the maximum power point

on the I-V curve are Imax and Vmax, respectively. The power conversion efficiency (PCE or

η) is the ratio of Pmax to the power of the incident light (Pin) and is given by the equation:

PCE = Pmax

Pin .

(2)

Another indicator of the overall quality of the cell is the fill-factor (FF), which is

represented graphically by the ratio of the areas depicted on the I-V curve. The fill factor

can be calculated by using Equation (3), which is given by:

FF = Imax ∙ Vmax

Isc ∙ Voc .

(3)

In the absence of light, a solar cell can be modelled as a diode, with current passing in one

direction only (see circuit diagram in Figure 1.7). When illuminated, the cell produces a

photo-generated current (Il). The total output current (I) for an ideal cell is given by

Equation (4) and is the difference of Il and the diode current (ID). Equation (4) is given by:

I = ID − Il = I0 (e(qV/nkT) − 1) − Il .

. (4)

In Equation (4), q is the elementary charge of an electron (1.6×10-19 C), k is the Boltzmann

constant (1.38×10-23 J/K) and T is the operating temperature in Kelvin.

12

Figure 1.7. Circuit diagrams for an ideal solar cell operating under dark and light

conditions and for a real cell, showing sources of parasitic resistances. By convention, the

anode and cathode are defined as the sites of hole and electron collection, respectively. The

“+” and “−” signs indicates the polarity of the applied bias during solar cell operation.

However, in a real solar cell, there are voltage losses due to parasitic series

resistance (RS) and parallel or shunt resistance (RSH). RS is related to the thickness of the

active layer, conductivity of the materials, electrode interfaces and carrier mobilities. RSH

is reflective of the amount of structural and morphological defects in the active layer;

defects decrease RSH, thereby decreasing photovoltaic performance.34 Taking these factors

into account expands the diode current equation to:

I = ID − Il = I0 (e(q(V+I∙RS)/nkT) − 1) − Il +V + I∙RS

RSH .

(5)

13

The effect of RS and RSH on the I-V curve is represented in Figure 1.8. RS and RSH can be

calculated as the inverse slope of the curve and open-circuit and short-circuit, respectively.

Figure 1.8. An illustration of I-V curves showing the detrimental effects on FF with a)

increasing RS and b) decreasing RSH.

1.5. Thin Film Formation for Solar Cell Applications

In the context perovskite and organic solar cells, a solid thin-film is formed after

nucleation of the material on a substrate and subsequent growth of “domains.” The

electronic, chemical and structural properties of the material in the thin-film are heavily

dependent on the method of deposition. As mentioned before, the most attractive

techniques use solutions of the dissolved material to produce the active layers (i.e.,

solution-processing). Among the myriad of solution deposition techniques that exist (and

have been reviewed in-depth),35 spin-coating is used almost exclusively for both laboratory

work and research and development on small-area substrates. While spin-coating is

generally not used in commercial applications, it is an ideal technique for high-throughput

screening of materials (as is the case for the projects undertaken in this thesis). Other

coating and printing methods such as slot die coating, spray-coating, screen printing and

14

ink-jet printing are compatible with roll-to-roll (R2R) printing, which can be employed on

large rolls of flexible substrates.

In the studies conducted herein, spin-coating is used exclusively as the method of

film-formation (see illustration in Figure 1.9). Spin-coating is accomplished by first

dispensing the solution onto the substrate. The substrate is then rapidly rotated up to a

desired rotational velocity or “spin-speed” (νrot), measured in revolutions per minute (rpm).

Most of the overlying solution is immediately ejected from the surface, leaving a thin “wet”

film of solution. As the solvent evaporates during the spin cycle, the material precipitates

or crystallizes on the substrate surface, forming a solid dry thin-film.

Figure 1.9. An illustration of the stages involved in the spin-casting procedure.

Two characteristics of the resulting spin-cast film that are critical to device

performance are: i) film thickness (h) and ii) morphology, or the physical arrangement of

the nano-scale domains. Both characteristics are largely dependent on the spin-coating

parameters, such as νrot, initial solution concentration (c0), viscosity (μ0), and density (ρ).

Since film thickness has been shown to depend on inverse square root of spin speed and is

15

directly proportional to c0, 36 h can be controlled relatively easily based on the relationship

given by Equation (6):

h ∝ (μ0

ρ ∙ νrot) 1/2 c0 .

. (6)

However, the morphology is often more difficult to predict than h. Therefore, controlled

experimentation and use of characterization tools to observe the thin-film morphology is

necessary to optimize the spin-coating conditions.

1.6. Project Goals

The overall goal of the research projects herein is to investigate the thin film

properties of the active layers for the production of low-cost and efficient perovskite (e.g.,

CH3NH3PbI3) and organic solar cells. The goal of the first project (Chapter 3) is to

fabricate an efficient perovskite solar cell. The methods involved in the deposition of

perovskite thin films from solution (i.e., the solution-processing conditions) are probed and

correlated to the morphology of perovskite thin films and the performance of the devices.

The second goal of the project (Chapter 4) is reduce the overall cost of the perovskite solar

cell by using cheaper organic hole-transporting materials. The third goal is to assemble a

low-cost non-fullerene donor-acceptor pair for the active layer of an organic solar cell

(Chapter 5). The effect of solution-processing techniques and molecular design on the

thin-film morphology, electronic properties and crystallinity are investigated for

complementary donor-acceptor pairs.

16

CHAPTER 2 – CHARACTERIZATION TECHNIQUES

In order to understand how materials form thin films in perovskite and organic solar

cells, techniques that elucidate the physical and chemical properties of materials in both

bulk form and in thin films are critical.37 This section describes the characterization

techniques and procedures that are applied to perovskite and organic materials and used

throughout this thesis.

2.1. Optical Microscopy (OM)

One of the simplest forms of microscopy is optical microscopy (OM). Since images

acquired by this technique are of low-resolution, it is limited to detecting micrometre-sized

topological film features for rapid screening. However, more powerful techniques are

required to view nano-sized film features. OM images were acquired under transmitted

light using a Zeiss Axio Imager and processed using the Zeiss Zen software package.

2.2. Scanning Electron Microscopy (SEM)

Scanning electron microscopy (SEM) allows the acquisition of higher-resolution

images than OM. A focused beam of electrons is raster scanned across the sample surface.

Secondary electrons are ejected from the sample and detected to generate a 2D image of

the film surface. In this thesis, SEM was used exclusively to image the surfaces of

perovskite films as a tool to detect pinholes and assess film coverage. All surface SEM

images were acquired using a Phenom G2 Pro bench-top SEM with backscatter detection,

in the laboratory of Prof. Jeffery Dahn.

2.3. Atomic Force Microscopy (AFM)

Atomic force microscopy (AFM) is advantageous over the aforementioned forms

of microscopy for the ability to elucidate the three-dimensional (3D) topography of the film

surface for quantification of domain sizes and height deviations. For this reason, AFM is

17

especially useful for imaging the surfaces of the active layers in organic solar cells for

which the domain sizes and the degree of D-A phase separation are critical to PV

performance. AFM is a form of scanning probe microscopy (SPM) in which a physical

probe traces across the sample’s surface, generating a 3D image (see Figure 2.1). As the

probe or “tip” is brought near the sample surface, interaction forces (e.g., chemical bonding,

Van der Waals, electrostatic forces, magnetic forces, etc.) between the sample and tip cause

the cantilever to deflect. On the Bruker Innova AFM used herein, the degree of deflection

is monitored by a laser.

A sample can be imaged in one of two ways: under constant height mode or constant

force mode. In both cases, a raster scan involves the tip tracing over the surface of the

sample in the xy plane. During a scan in constant height mode, the z position of the

cantilever is fixed. Changes in the deflection signal are monitored using a piezoelectric

crystal and are recorded as a function of position. In constant force mode, signal from a

feedback controller is applied to a piezoelectric crystal which moves the position of the tip

(z) to maintain constant cantilever deflection. All experiments herein were performed using

constant force mode.

Figure 2.1. An illustration of the operation of an AFM.

18

There are two common modes of AFM topography: contact and tapping mode.

Contact mode is simpler; the cantilever deflection is static as the tip traces over the sample.

In contrast, during tapping mode, the cantilever oscillates at a resonant frequency and the

amplitude of the oscillation is maintained by the feedback controller. Since the tip

alternately contacts and lifts from the sample surface during tapping mode, frictional forces

are reduced; the result is a less-destructive technique.

AFM experiments were conducted in tapping mode using a Bruker Innova

microscope and NanoDrive (v 8.02) software. The AFM instrument was located in the

laboratory of Prof. Ian Hill. Square images of 5 × 5 µm in dimension were acquired by

scanning at a rate of 0.3 Hz and sampling 256 points per line. Each image was processed

using the Nanoscope Analysis software package.

Three parameters can be used to quantify the surface morphology of a thin film

using the Surface Roughness Tool in the NanoDrive program: i) the 3-dimensional image

surface area, ii) RRMS, the root mean square (RMS) average of height deviations relative

to the mean image data plane, iii) Ra, the average of the absolute values of the surface

height deviations relative to the mean plane, and iv) Rmax, the vertical distance between

the highest and lowest data points in the image. Zi is the deviations of the height from the

mean data plane in the ith pixel of the image. N is the number of pixels in the image.

RRMS and Ra are calculated from the following expressions:

RRMS = (∑ Zi

2

N)1/2 , and

(7)

19

𝑅a = 1

N ∑ |Zi|

N

i = 1

.

(8)

2.4. X-ray Diffraction (XRD)

XRD experiments are used to: i) identify the materials present and ii) assess the

crystallinity of materials within the thin films. Insight into the arrangement of molecules

within the crystallites in the films can be gained using Bragg’s Law:

nλ = 2dsinθ (9)

where n is an integer, λ is the wavelength of incident radiation from the instrument, θ is the

angle of incidence of X-rays with respect to the atomic planes of the crystal and d is the

separation distance of the atomic planes (i.e., spacing). The Rigaku diffractometer used for

the XRD experiments has a Cu-Kα radiation source, so the X-rays produced have a

wavelength of 1.54 Å.38

XRD patterns were acquired using a Rigaku Ultima IV X-ray diffractometer

equipped with a CuKα radiation source, scintillation detector, fixed monochromator and

285 nm focusing slit. The film-coated substrates were mounted into the instrument on a

metal platform. Experiments were run using the RINT2200 Right software package with

divergence and receiving slit widths of 10 mm and 0.3 mm, respectively, and an offset

angle of 0°. Powder patterns were acquired by continuous scans, collecting data as counts

per second (cps) and scanning at a rate of 2° per minute. The patterns in each data set were

normalized such that the level of the noise is equivalent, that is, the intensity of the pattern

at a Bragg angle of 3.05°.

2.5. UV-Visible (UV-Vis) Spectroscopy

20

UV-Visible spectroscopy (UV-Vis) can be performed on conjugated organic

molecules in solid-state thin films or dissolved in solution. The absorption profile provides

information about the energy of the light absorbed by the material, which is related to the

bandgap energy (Eg), or the difference between the HOMO and LUMO energy levels. Eg

is determined from the wavelength of the onset of absorption (λonset), or the longest

wavelength of visible light that is absorbed by the material, and applying Equation (10):

Eg =h∙c

λonset .

(10)

In Equation (10), h is the Planck constant (4.136×10−15 eV∙s) and c is the speed of light

in vacuum (2.998×108 m/s). All UV-Vis spectra were acquired using an Agilent Cary 60

spectrophotometer with the Cary Scan software program. For thin-film experiments, the

film-coated glass substrates were mounted vertically into the instrument with the glass side

facing the radiation source. The instrument was first zeroed using a clean glass slide as a

blank. Spectra were acquired by scanning from 900 nm to 200 nm at a rate of 600 nm/min.

The spectra were normalized to the peak absorbance value. All solution samples were

prepared by stirring a weighed quantity of each material in a vial with CHCl3 or

chlorobenzene (CB) as the solvent until dissolved. After transferring the solutions into

quartz cuvettes, UV-Vis spectra were acquired by scanning from 900 nm to 200 nm at a

rate of 600 nm/min.

2.6. Photoluminescence (PL) Spectroscopy

The term photoluminescence (PL) refers to the emission of photons following

photoexcitation. Photoexcitation occurs when electrons are promoted to higher-energy

states upon absorption of light. As the electrons relax to the ground-state via various

21

mechanisms, photons can be re-radiated and detected by the PL instrument. During PL

experiments, the sample is irradiated by visible light of a particular wavelength (excitation

wavelength, λex) and an emission spectrum is recorded.

The film-coated substrates were mounted vertically into the instrument with the

film-side facing the radiation source at a 45° angle. For emission experiments, λex was

selected as the wavelength at which maximum absorption for the particular material occurs

in a thin film (determined from the results of thin-film UV-Vis experiments). Excitation

and emission slit widths were set to 20 nm. Spectra were acquired by scanning at a rate of

600 nm/min.

2.7. Cyclic Voltammetry (CV)

CV was used to measure the onset of oxidation and reduction of the materials, which

corresponds to the ionization potential (HOMO) and electron affinity (LUMO),

respectively. This information is critical in D-A organic solar cells, where the efficient

charge separation at the D-A interface depends on appropriate alignment of the HOMO and

LUMO energy levels.

CV experiments for organic materials were conducted using solution samples. A

BASi CV instrument was equipped with an N2 bubbler as well as an Ag/AgCl electrode, Pt

wire and glassy-carbon electrode as the pseudo-reference, counter electrode and working

electrode, respectively. Measurements were conducted using the BASi Epsilon EC

software program. Samples were prepared with a concentration of ~1 mg/mL by dissolving

each material in anhydrous CH2Cl2 with ~0.1 M tetrabutylammoniumhexafluorophosphate

(TBAPF6) as the supporting electrolyte. All solutions were purged with N2(g) and then

scanned at 50, 100 and 150 mV/s as-is and at 100 mV/s after the addition of a ferrocene

22

(Fc) standard. The resulting voltammograms were referenced to the oxidation potential of

Fc/Fc+, which corresponds to the HOMO energy level at a value of 4.80 eV below the

vacuum level.39 The values of the HOMO and LUMO energy levels of the organic

compounds were obtained by comparing the onset of oxidation and reduction, respectively,

to the HOMO energy of ferrocene.

23

CHAPTER 3 – THIN FILM FORMATION IN PEROVSKITE SOLAR CELLS

3.1. Background

3.1.1. Solution-Processed Perovskite Solar Cells

The combination of interesting material properties and the reported high

performance of perovskite solar cellss make perovskites an attractive field of study. For

this reason, it was a collective decision to develop a research program in the Welch and

Hill laboratories focused on the development and characterization of materials for PSCs.

Before the study of novel materials could be conducted, it was necessary to develop

an in-house fabrication procedure for reproducible and high-efficiency standard devices to

serve as controls. While many reports in the literature exist, fabricating a PSC is not a

straight-forward process and ultimately depends on film formation of the active layer. This

chapter serves as an account of the challenges encountered during the development of a

functional perovskite solar cell.

3.1.2. Active Layer Components and Device Architecture

There are two general types of active layers for PSCs: Meso-structured and planar

heterojunction (PHJ) (see Figure 3.1). In mesostructured PSCs the perovskite material is

housed within a mesoporous scaffolding of semiconducting (e.g., TiO2)17 or insulating

(Al2O3)40,41 nanoparticles. In contrast, a planar heterojunction perovskite solar cell does

not contain the mesoporous oxide layer; the active layer is simply an unsupported thin film

of perovskite material.42,43 While preliminary mesoporous devices were fabricated (results

are not reported in this thesis), the PHJ architecture was selected for further optimization

due to its simplicity in construction.

24

Figure 3.1. Device architectures for planar and meso-structured perovskite solar cells.

For planar perovskite solar cells, a mixed halide perovskite (CH3NH3PbI3-xClx) is a

preferred material for planar architectures because of the longer electron-hole diffusion

length44 than its pure-halide45 counterpart (CH3NH3PbI3). Nevertheless, pure-halide (i.e.,

MAPbI3) perovskites have been used in planar cells with high efficiencies.24,46 For this

reason, MAPbI3 is the perovskite material used in the studies throughout this thesis.

Regardless of the material selection, film uniformity of the perovskite active layer

is critical for high performance devices and is difficult to achieve. It has been demonstrated

that controlled vapour deposition is an effective method that can achieve uniform films and

highly efficient planar cells with both mixed- and pure-halide perovskite active layers.24,47

However, this method is expensive and challenging on an industrial scale. On the other

hand, solution-processing is a cost-effective method for active layer deposition and is

necessary to realize large-scale R2R production of perovskite solar cells.48

3.1.3. Methods of Solution-Processed Perovskite Thin Films

One of the most important factors that influences the performance of a planar

heterojunction perovskite solar cell is the active layer film morphology (e.g., crystal grain

25

size, crystal distribution, overall uniformity and coverage, absence of pinholes, etc.).

Furthermore, the film morphology is largely dependent on the processing methods and

conditions applied during film formation.

Three well-documented methods exist for perovskite film formation from solution:

i) Co-Deposition. The perovskite is formed in-situ by spin-coating a single

solution containing the two constituents (i.e., methyl ammonium iodide

(CH3NH3I or “MAI”) and lead (II) iodide (PbI2)).17,49

ii) Sequential Dip-Coating (SDC). A film of the metal halide is first deposited on

the substrate via spin-coating and then dipped into a solution containing the

ammonium salt. In this method, (which was first documented by Liang et al),50

PbI2 and MAI react to form the final perovskite film. This method has proven

to be an effective active-layer processing technique in perovskite solar cells,

helping to fabricate devices that achieve over 15% efficiency.46,51 In addition,

modifications to the two-step method to control PbI2 crystal growth via solvent

engineering have been discovered that yield devices with over 16%.52

iii) Sequential Spin-Coating (SSC). Similar to the SDC method, a thin film of PbI2

is first deposited onto the substrate.53 Subsequently, a film of MAI is spin-

coated on top of the PbI2 layer. Heating the layers stimulates the interdiffusion

of MAI ions into the underlying PbI2 film, forming the perovskite, MAPbI3.

The first goal in this project was to screen previously-reported methods for perovskite

film formation. However, processing conditions vary from lab to lab – the optimal

conditions for film formation reported in the literature are not universal. Furthermore, it

was found that many subtle techniques and “tricks-of-the-trade” are often excluded from

experimental sections of “high-impact” papers. For these reasons, it was important to

26

identify variables and perform controlled sets of optimization experiments to develop a

custom in-house method for perovskite film formation. Methods for both SDC and SSC

were developed and discussed herein.

3.2. Sequential Dip-Coating Experimental Methods

3.2.1. PbI2 and MAPbI3 Thin Films

Thin films of PbI2 and MAPbI3 were prepared via spin-coating on TiO2-coated glass

substrates to determine the optimal processing conditions. The procedure for making these

films was as follows.

25 mm × 25 mm glass slides were first cleaned in the following order: i) scrubbing

with mixture of deionized (DI) H2O and Sparkleen® detergent, ii) rinsing with DI H2O, iii)

rinsing with acetone, iv) rinsing with isopropanol, v) drying under a stream of air and iv)

UV/ozone treatment for 15 min.

The clean substrate was first mounted onto the chuck of a Laurell spin-coater. To

form the dense TiO2 layer, the precursor solution (see Appendix A for preparation

procedure) was dispensed onto the center of the 625-mm2 substrate through a 0.45 μm

PVDF filter using a 1 mL syringe in one aliquot of 0.3 mL. The film was then spun at 2000

rpm (ramp = 2050 rpm/s) for 45 seconds. The coated substrates were placed in clean

Pyrex® petri dishes and then placed in a sintering oven. The temperature was ramped up

to 500 °C at a rate of 30 °C/min and held at 500 °C for 30 min. The films were then

removed from the oven and cooled to room temperature (RT).

To form the PbI2 films, PbI2 solutions with N,N-dimethylformamide (DMF) as the

solvent were prepared (see Appendix A for preparation procedure). At a concentration of

1.0 M, the solution was supersaturated at RT and had to be cast at an elevated temperature.

The dilute solutions (i.e., 0.5 M and 0.75 M) were cast at RT.

27

The TiO2-coated glass substrates were used either heated (on a hotplate) or unheated

(at RT). After placing the substrate onto the chuck of the spin-coater, approximately 0.3

mL of solution was retracted from the vial using a 1.0 mL syringe and dispensed onto the

center of the substrate through a 0.45 µm PTFE filter in a single aliquot. The spin-coater

was then run at speeds of either 1000, 3000 and 5000 rpm for 30 s. For heated substrates,

the average elapsed time from removal of the substrate from the heat source and solution

deposition was 30 s.

The dipping solutions were prepared by weighing the dried MAI (dried in a vacuum

oven overnight prior to solution preparation) into a 30 mL beaker, adding the appropriate

amount of isopropanol to form a 10 mg/mL solution and then stirring the solution at RT

(using a stir bar and a hotplate) for approximately 15 min. The synthesis of MAI is

described in Appendix B. To initiate the conversion into a MAPbI3 perovskite film, the

PbI2 films were briefly rinsed a beaker containing isopropanol, removed, and then

immersed in the beaker containing the MAI solution. The beaker was gently swirled by

hand for 60 s to promote uniform diffusion of the MAI across the entire PbI2 film surface.

After 60 s, the substrate was removed from the dipping solution using tweezers, rinsed in a

beaker of isopropanol and finally dried under a stream of filtered air or N2. The perovskite

films were characterized as-is without further processing and stored in an Ar-filled

glovebox. The SDC procedure is illustrated in Figure 3.2.

28

Figure 3.2. Photographs depicting the stages of the SDC procedure.

3.2.2. Perovskite Solar Cell Fabrication Using Sequential Dip Coating

The 25 mm × 25 mm FTO-glass substrates were first cleaned by the following

procedure: i) scrubbing the surface with lab detergent (Sparkleen®) and DI H2O, ii)

sonicating in a solution of DI H2O and Sparkleen® for 15 min, iii) sonicating in acetone

for 15 min, iv) sonicating in ethanol for 15 min, iv) rinsing in DI H2O and v) UV/ozone

treatment for 20 min.

The TiO2 film was deposited immediately following UV/ozone treatment using the

method described in Section 3.2.1. Before entering the oven, the four corners of the

substrate were carefully wiped with a CleanSwab® dampened with 37% HCl to expose the

underlying FTO. The substrates were then placed in a Pyrex® petri dish and then baked in

the sintering oven at 500 °C for 30 min. After cooling to room-temperature, the substrates

were either heated on a hotplate to establish the desired Tsubstrate or used unheated.

Only 1.0 M PbI2 solutions were used to make PbI2 films for PSCs. For deposition

on heated substrates, the substrates were removed from the hotplate and quickly placed

onto the chuck of the spin-coater. Approximately 0.3 mL of the hot PbI2 solution was

retracted into a 1.0 mL syringe and then quickly dispensed onto the center of the heated

substrate through a 0.45 µm PTFE filter in one aliquot. The spin-coater was then run at

29

speeds of either 1000 or 3000 rpm for 30 s. The four corners were then wiped with

CleanSwab® dampened with DMF to expose the underlying FTO. The PbI2 films were

converted into MAPbI3 in all devices following the procedure outlined in Section 3.2.1.

To form the hole-transporting layer, the solution containing p-doped spiro-

OMeTAD (see Appendix A for preparation procedure) was dispensed onto the perovskite

layer through a 0.45 µm PTFE filter and then spun at a rate of 4000 rpm for 30 s under

ambient conditions. The four corners were then wiped with a CleanSwab® dampened with

chlorobenzene to expose the underlying FTO.

The top metal contact was deposited via thermal evaporation. The substrates were

loaded top-down into a metal mask and then mounted in a custom-built bell-jar thermal

evaporator. A silver slug was used as the metal source in a tungsten-wire basket. The bell-

jar was evacuated using a diffusion pump to a pressure less than 4×10-6 Torr. A 75-nm

thick layer of silver was deposited onto the substrates by thermal evaporation. After

removal from the bell-jar, the devices were immediately transferred into an Ar-filled

glovebox where they were tested and subsequently stored.

3.3. Perovskite Solar Cell Device Characterization

For photovoltaic measurements, solar cell devices were illuminated using a

calibrated light source. A National Institute of Standards and Technology (NIST)-traceable

calibrated photodiode (Newport 818-UV-L) in conjunction with a near infrared absorptive

filter (Thorlabs NENIR60A) was used to measure the output power density of the Xe arc

lamp (Sciencetech SS0.5K) below 800 nm, where the spectrum of the source closely

matches the solar spectrum and covers the absorption range of all photoactive materials

used in this thesis. This provided a power density of 100 (±1) mW/cm2 of AM 1.5G

spectrum over relevant wavelengths, or the equivalent of 1.00 Suns. Each device had an

30

area of 0.032 cm2 and was tested in separate measurements. One exposed FTO corner was

contacted with the negative (black) electrode and the Ag top contact of a device was

contacted by gently approaching a thin gold-wire that was connected to the positive (red)

electrode (illustrated in Figure 3.3). Current-voltage (I-V) curves were then measured

using a Keithley source-measure unit, scanning from +2.0 V to short-circuit (0.0 V) unless

otherwise stated.

Figure 3.3. A photograph of a substrate with 16 completed PSC devices. The points of

contact to the positive and negative electrode are indicated by red and black arrows,

respectively.

3.4. Results and Discussion – Sequential Dip-Coating

While investigating the sequential dip-coating (SDC) method for perovskite thin

film formation, it was found that the morphology of the final perovskite film is largely

dependent on the morphology of the initial PbI2 film. A major issue was the formation of

large, needle-like crystals of PbI2, which formed cloudy, non-continuous films with poor

substrate-coverage, unsuitable for the active layer of PSCs. Perovskite active layers with

maximum surface coverage are desired in devices to prevent shunt pathways and short-

31

circuits. Therefore, methods that generate dense perovskite layers with a uniform

distribution of crystallites are favoured.

Rapid cooling of the hot solution during spin-coating of the PbI2 was identified as

a cause for the large-crystal growth. Various spin-coating parameters were systematically

controlled to investigate the effect on film morphology. The solution concentration

(Csolution), spin speed (νrot), and substrate temperature (Tsubstrate) were found to have the most

significant effect on film morphology. Conditions including the solution temperature

(Tsolution, Figure C1) and atmosphere (Figure C2) were also investigated and are discussed

in Appendix C.

3.4.1. Solution Concentration (Csolution) and Spin Speed

The most common solution used in the literature for the deposition of the PbI2 film

during the two-step perovskite deposition method is a 1.0 M (461 mg/mL) solution in DMF.

While this solution has proven to yield thick, uniform films in devices with high

efficiencies, its use is problematic: it is a supersaturated solution at room temperature and

must be deposited at elevated temperatures (i.e., “hot-cast”). Following this procedure,

there is a tendency for the PbI2 to rapidly crystallize on the substrate before the film is spun.

By reducing the concentration, it is possible to deposit a homogeneous solution of PbI2

without the need for hot-casting. Figure 3.4 depicts OM images of PbI2 films deposited

from solutions of different concentrations and different spin speeds onto substrates held at

room temperature. It was observed that film uniformity and coverage increases with

increasing spin velocity – a phenomenon that was also observed by Cohen et al.54 The

crystalline domain sizes decrease with increasing spin velocity. This is attributed to the

higher rate of solvent evaporation as the spin speed is increased. From the optical

microscope images, it is clear that perovskite films cast from 1.0 M solutions provide better

32

substrate coverage and are more desirable as active layers in PSC devices than those cast

from more dilute solutions (Figure 3.5). For this reason, all further modifications to

processing conditions in this study were performed using 1.0 M solutions.

Figure 3.4. Optical microscope images of PbI2 films spin-cast on TiO2 compact films

showing the effect of precursor-solution concentration. The 0.5 and 0.75 M solutions were

deposited at room temperature and spun for 90 s. The 1.0 M solution was deposited at 70

°C and spun for 30 s. All films were cast onto room-temperature substrates.

Figure 3.5. SEM images of the CH3NH3PbI3 films after conversion of the PbI2 films via

SDC, showing the effect of precursor-solution concentration.

33

3.4.2. Substrate Temperature (Tsubstrate)

To reduce the rate of cooling of the solution after dispensing it onto the cold

substrate, films were cast onto heated substrates (from 50 °C up to 150 °C) and compared

to those cast onto unheated substrates (Figure 3.6). It is important to note that the substrate

cools considerably between the time of heating and the time when the solution is dispensed.

To determine the substrate temperatures (Tsubstrate), a thermocouple probe was attached to

the surface of the substrate using Kapton® tape. The initial substrate temperature

(Tsubstrate,initial) was recorded while the substrate was resting on the hotplate with a surface

temperature of Thotplate. These measurements were recorded after at least 10 minutes of

heating. The final substrate temperature (Tsubstrate,final) was recorded 30 s after the substrate

was removed from the heat source and placed on the chuck of the spin-coater. The

measurements for substrate temperatures are listed in Table 3.1.

It was observed that glass microscope slides, which are considerably thinner than

the FTO-coated glass substrates used for device fabrication, have a higher rate of cooling.

For instance, if a glass microscope slide and an FTO-coated glass substrate are heated to

100 °C and 75 °C, respectively, both will cool to a temperature of 60 °C after being placed

on the chuck of the spin-coater for 30 s (i.e., Tsubstrate,final is ~ 60 °C).

Table 3.1. Initial and final (i.e., after 30 s) temperatures of glass and FTO substrates heated

on a hotplate set to a surface temperature of Thotplate.

Tsubstrate,initial

(°C)

Thotplate

(°C)

Tsubstrate,final

(°C) (Glass)

Tsubstrate,final

(°C) (FTO)

50 66 40 46

75 92 49 59

100 117 60 69

125 135 65 85

150 160 73 105

34

Figure 3.6 depicts the films cast on substrates heated to 100 °C just prior to

deposition are more uniform and consist of smaller domains than those cast on room-

temperature substrates. The additional heating step in the processing procedure is

beneficial for two reasons: First, water is removed from the surface on the substrate,

allowing better contact with PbI2 ions and nucleation sites. Second, the heated substrate

helps to maintain the elevated temperature of the supersaturated solution, thus preventing

rapid cooling of the solution and corresponding crystallization of the solute. For example,

it is observed that the films spun on unheated substrates at 1000 rpm consist of large needle-

shaped crystals, whereas the films cast on heated substrates consist of significantly smaller

grains. By spin-coating at a slow speed and heating the substrate (e.g., 1000 rpm), thicker

films can be produced that provide comparable coverage to films spun at higher speeds.

Figure 3.6. The effect of spin velocity and substrate temperature. SEM images of PbI2

films spin-cast on TiO2 compact films at different temperatures and the corresponding

CH3NH3PbI3 films after conversion via dipping. All films were spin-cast from a 1.0 M

PbI2 solution in DMF held at 70 °C and spun for 30 s.

35

Although the film cast on a R.T.-substrate consists of needle-like crystals and large

voids, substrate temperatures of 50 °C and above resulted in more uniform surface coverage

and no needle growth (see Figure 3.7). Intermediate temperatures (50 and 75 °C) resulted

in a honeycomb PbI2 morphology whereas the films cast on the substrates at the highest

temperatures (100, 125, 150 °C) consisted of larger grains. After conversion to the

perovskite via dipping into the MAI solution, the films cast on all substrates heated to

temperatures above 50 °C display similar morphologies that consist of micron sized

cuboidal crystallites.

Figure 3.7. The effect of substrate temperature. SEM images of PbI2 films spin-cast on

TiO2 compact films at different temperatures and the corresponding CH3NH3PbI3 films

after conversion via dipping. All films were spin-cast from a 1.0 M PbI2 solution in DMF

held at 70 °C at a spin speed of 3000 rpm for 30 s.

3.4.3. Device Performance Using Sequential Dip Coating

Based on the optical microscope and SEM images acquired for the PbI2 films, it is

clear that substrate temperature has a significant effect on morphology. To correlate the

morphological changes to PV performance, PHJ perovskite solar cell were fabricated on

substrates heated to a range of temperatures.

36

When keeping the spin speed constant at 3000 rpm, increasing the temperature of

the substrate decreases cell performance in terms of efficiency and current density (Figure

3.8 and Table 3.2). However, the devices with PbI2 films cast at 1000 rpm benefit from

substrate heating (Figure 3.9 and Table 3.3). By heating the substrate to 100 °C prior to

deposition, the resulting devices display a higher shunt resistance. This is in agreement

with the optical microscope images of the PbI2 and perovskite films; films cast on the heated

substrates show visibly greater coverage and correspondingly higher RSH.

Figure 3.8. a) J-V curves for the best-performing PSCs with PbI2 films deposited at 3000

rpm on substrates heated and unheated substrates. b) A plot of the average PCE of PSC

devices with PbI2 films deposited at 3000 rpm as a function of substrate temperature. For

each substrate temperature, error bar range extends from the lowest- to the highest-observed

PCE among five representative devices.

Table 3.2. J-V performance metrics for the best-performing PSCs fabricated with PbI2

films deposited at 3000 rpm on substrates heated and unheated substrates. The average

metrics for five sample devices (at each substrate temperature) are also provided with

standard deviations in the brackets.

νrot (rpm) Tsubstrate

(°C) Voc (V) Jsc (mA cm-2) PCE (%) FF

3000 R.T. Best Cell 0.97 −18.20 9.76 0.56

Average 0.93(0.02) −18.12(0.26) 7.48(2.01) 0.44(0.11)

3000 100 Best Cell 0.93 −14.52 5.31 0.39

Average 0.92(0.01) −14.43(0.56) 4.99(0.42) 0.38(0.02)

3000 150 Best Cell 0.62 −8.67 1.91 0.36

Average 0.56(0.07) −8.79(1.01) 1.76(0.14) 0.36(0.00)

37

Figure 3.9. J-V curves of the best-performing PSCs with PbI2 films deposited at 1000 rpm

on substrates heated to different temperatures. OM images of the active layers are also

shown.

Table 3.3. J-V performance metrics of the best-performing PSCs with PbI2 films

deposited at 1000 rpm on substrates heated to different temperatures. The average

metrics for five sample devices (at each substrate temperature) are also provided with

standard deviations in the brackets. The shunt resistances listed are for the best-

performing devices only.

νrot

(rpm)

Tsubstrate

(°C) Voc (V) Jsc (mA cm-2) PCE (%) FF

Shunt

resistance

(Ω cm-2)

1000 RT Best Cell 0.75 −11.39 2.86 0.34 9.0×10-4

Average 0.72(0.08) −9.32(2.85) 1.99(0.88) 0.28(0.03) -

1000 100 Best Cell 0.99 −11.67 3.94 0.34 2.1×10-5

Average 0.89(0.03) −10.54(2.92) 2.67(1.09) 0.27(0.04) -

3.5. Experimental Methods – Sequential Spin Coating

Inconsistencies in device performance using the SDC method led to an investigation

of an alternative method of perovskite film formation: sequential spin-coating (SSC), which

is illustrated in Figure 3.10.

Figure 3.10. Photographs of films depicting stages of the SSC procedure.

38

TiO2 films were first prepared on cleaned glass substrates as per the procedure

outlined in Section 3.2.1. After the substrates were cooled to RT, PbI2 films were spin-

coated onto the TiO2-coated substrates by dispensing the hot (70 °C) PbI2 solution (1.0 M

in DMF) onto the substrate through a 0.45 μm PTFE filter using a syringe and then spinning

at 6000 rpm for 60 s. The substrates were immediately removed from the spin-coater and

placed on a hotplate with a surface temperature of 70 °C for 5 min to remove excess DMF.

The substrates were cooled to RT before depositing the MAI layer.

MAI solutions of varying concentrations were prepared dissolving dry MAI crystals

in isopropanol in 4 mL glass Teflon-capped vials and stirring at room temperature. MAI

films were spin-coated onto the dry PbI2 films by dispensing the MAI solution onto the

substrate through a 0.45 μm PVDF filter using a syringe and then spinning at 6000 rpm for

60 s. The PbI2/MAI layered films were then annealed on a hotplate at 100 °C for 1 h. Films

for all PSC devices were annealed in an Ar-filled glovebox, for reasons that are explained

in Section 3.6.1.

3.6. Results and Discussion – Sequential Spin Coating

3.6.1. Annealing Conditions

After both the PbI2 and MAI layers have been deposited, heat is required to drive

the interdiffusion of MAI into the lattice of the underlying PbI2. Lead iodide can be

described as a “quasi two-dimensional” material since the 2D atomic planes of lead iodide

are arranged in layered stacks.55 The spaces between the atomic planes have been shown

to facilitate the intercalation of small molecules, such as methylammonium iodide.52 To

confirm the presence of CH3NH3PbI3 in the thin films, the experimental XRD patterns were

compared against a simulated pattern of CH3NH3PbI3 obtained from the crystal structure

data reported by Stoumpos et al. (see Figure 3.11). Annealing under inert atmosphere at

39

100 °C for 1 hour results in full conversion to the perovskite, as indicated by absence of the

110 PbI2 peak (2θ = 12.6°) in the thin-film XRD patterns Figure 3.11. On the other hand,

the films that were annealed in ambient conditions for 1 hour contained unconverted PbI2.

It is possible that the moisture in air contributes to degradation of the perovskite into lead

iodide, which has been demonstrated in a previous study by Kelly et al.56 No significant

differences were observed in terms of morphology for the films annealed in ambient versus

those annealed in inert atmosphere (see Figure 3.12). Based on these results, all subsequent

perovskite films were annealed in an Ar-filled glovebox.

Figure 3.11. XRD pattern of a PbI2 thin film on TiO2 compared to CH3NH3PbI3 films after

annealing at 100 °C for 1 h in inert (black trace) or ambient (grey trace) atmosphere. The

simulated powder pattern (blue trace) was obtained from the CH3NH3PbI3 crystal structure

data (in the form of a cif file) reported by Stoumpos et al.18 The corresponding lattice

planes for each of the diffraction peaks in the PbI2 pattern ((001), (002) and (003)) were

identified according to Burschka et al.51 The peaks corresponding to the major lattice

planes in the CH3NH3PbI3 patterns ((110), (220) and (330)) were identified based on the

analysis performed by Dualeh et al.57

40

Figure 3.12. 2D AFM topographical maps of the surface of two perovskite films formed

by the sequential spin-coating method – one was annealed in ambient and one annealed in

an inert atmosphere.

3.6.2. MAI Concentrations

In order to establish the correct stoichiometry between the PbI2 and MAI layers for

complete perovskite conversion, a series CH3NH3PbI3 films were formed by sequential spin

coating using different concentrations of MAI solution. For all films, the PbI2 spin speed

(6000 rpm), PbI2 solution concentration (1.0 M), MAI spin speed (6000 rpm), PbI2-MAI

annealing time (60 min) and annealing temperature (100 °C) were kept constant.

XRD was used to detect the phases of the materials present in the final films (Figure

3.13). The intensity of the (110) PbI2 peak (2θ = 12.6°) decreased linearly with increasing

MAI concentrations, indicating that less PbI2 remains unreacted in the film (Figure 3.14).

Visually, the films with a higher MAI loading appear darker and more optically dense due

to the higher quantities of MAPbI3 (see photographs of films in Figure 3.13). As shown

in the XRD patterns in Figure 3.13, perovskite thin films spin-coated with MAI solutions

less than 50 mg/mL all contained (001), (002) and (003) PbI2 diffraction peaks of

significant intensity. A plot of the relative intensity of the (001) PbI2 peak in the perovskite

powder patterns as a function of MAI concentration is provided in Figure 3.14 and shows

41

a linear decrease in peak intensity with increasing MAI concentration. This provides

evidence for presence of unreacted starting material and incomplete perovskite conversion

when using dilute MAI concentrations.

Figure 3.13. XRD patterns of MAPbI3 thin film formed via the sequential spin-coating

method with different concentrations of MAI precursor solution. All films were annealed

at 100 °C under inert atmosphere after spin-coating the MAI and PbI2 layers. A powder

patterns of a PbI2 neat film is included at the bottom of the plot for comparison. Also shown

are photographs of the MAPbI3 films, showing progressive darkening of the film with

increasing MAI concentration.

42

Figure 3.14. Relative XRD peak intensities for the highest-intensity peak of a) PbI2 and b)

MAI plotted as a function of MAI precursor concentration. The relative peak intensity was

determined by dividing the integrated intensity of the (001) PbI2 peak (IPbI2) by the sum of

IPbI2 and the intensities of the MAPbI3 (110) peak (2θ = 14.1°). Areal peak intensities were

calculated by applying a Pearson VII function using Jade software. Although error bars

are included in the plot, they are not visible. Measures of uncertainty associated with the

calculated peak intensities are listed in Table D1 in Appendix D.

3.6.3. Device Performance Using Sequential Spin-Coating

Perovskite solar cell devices were fabricated to determine the optimal MAI

precursor solution concentration for the sequential spin-coating method. All other

components in the devices were fabricated following the same procedures used for the

devices made via the dip method. Devices made with the higher loading of MAI (50

mg/mL) were more efficient as indicated by the J-V curves in Figure 3.15. These results

are comparable to devices made using similar procedures, in which the reported PCEs of

sample devices are 13.4 %58,59 and 15.4 %.53 Comparing the J-V results to the XRD

43

patterns in Figure 3.13, the higher device performance in the devices fabricated with a

more concentrated MAI solution is attributed to the more complete perovskite conversion.

Figure 3.15. J-V curves for the best-performing PSC devices fabricated via the SSC

method using an MAI concentration of 40 mg/mL (red curve) and 50 mg/mL (blue curve).

Performance metrics for the best-performing device and the average of five sample devices

are given in the table to the right of the curves. The values in brackets are the standard

deviations.

3.7. Conclusions

It has been demonstrated that simple adjustments in processing parameters can have

large effects on the morphology of perovskite thin films during both dip-coating and spin-

coating procedures. In particular, solution concentration and substrate temperature are key

variables to monitor during the dip method. The dipping step in the SDC method was

difficult to reproduce for two reasons. First, the conversion rate from PbI2 to perovskite

varied from batch-to-batch. Second, drying of the film with the air gun led non-uniformity

across the substrates. These factors led to batch-to-batch inconsistencies and even

variations in performance among the devices on the same substrate.

In contrast, the sequential spin-coating method allowed more control over conversion

of the perovskite and morphology of the film, producing more consistent films and high-

performing devices. Therefore, the spin-coating method was selected for use in subsequent

44

device fabrications as the project moved forward to the next chapter: investigating new

hole-transporting materials in PSCs.

It is important to consider the thermal properties of CH3NH3PbI3 when investigating the

two aforementioned methods of perovskite film formation. CH3NH3PbI3 has two known

phase transitions at temperatures of 162.7 K (~110 °C) and 326.6 K (~54 °C).60,61 Since

the perovskite made using the dip method is not heated during or after the conversion

process, it remains in a single phase. In contrast, the perovskite is heated above its upper

transition temperature (54 °C) following the sequential spin-coating procedure, resulting in

a phase transition that marked by a significant crystal lattice volume expansion.62 The work

by Bi et al.59 suggests that prolonged heating of a CH3NH3PbI3 thin film above the upper

transition temperature (e.g., at 100 °C) promotes the growth of larger crystallites. The

result is a compact and continuous perovskite active layer with devices showing improved

device FF and PCE compared to those that were heated for shorter durations. This work

provides evidence for beneficial changes in morphology upon heating above the

CH3NH3PbI3 and may explain the superior performance of devices made via the sequential

spin method over the dip method.

45

CHAPTER 4 – ORGANIC HOLE-TRANSPORTING MATERIALS FOR

PEROVSKITE SOLAR CELLS

4.1. Background

Achieving a robust and reproducible method for the fabrication of efficient planar

heterojunction solar cells was an important outcome of Chapter 3. Although high

efficiency is among one of the top priorities in the development of PSCs, another important

consideration is the cost. The goal for Chapter 4 is to investigate cheaper alternatives for

organic hole-transporting materials in perovskite solar cells.

Spiro-OMeTAD was the first solid-state organic hole-transporting material used in

perovskite solar cells17 and continues to be the industry-standard. However, spiro-

OMeTAD is expensive due to the multifaceted synthesis of its starting materials.63 It is

therefore important to develop economical HTMs synthesized from low-cost starting

materials to reduce the overall cost of PSCs.

Design of a new organic HTM requires the piecewise attachment of π-conjugated

functional groups. Ideally, a cheap building block, such as triphenylamine (TPA), is at the

centre or “core”. The TPA core adopts a pseudotetrahedral three-dimensional (3D)

geometry (see Figure 4.1 a)).64 This structural motif has been proven to yield effective

hole conductors for a variety of organic electronic applications.65–69 Each of the three

phenyl groups can be functionalized at para positions forming the three “side-arm” units.

The result is a 3D propeller-shaped molecule. This design motif has proven to be successful

for other TPA-based HTMs in PSCs, yielding high efficiencies (see Figure 4.1).70,71

46

Figure 4.1. a) A structural representation of the design motif for HTMs with a TPA core.

Two examples of propeller-shaped small-molecule HTMs used in PSCs ((b)70 and (c)71)

are also shown.

Using the motif depicted in Figure 4.1 a), side-arms can be designed with different

functionalities to build TPA-based derivatives with different electronic properties.72 Three

functional groups commonly used for organic π-conjugated materials are thiophene, furan

and phthalimide (shown in Figure 4.1); these groups can be adopted in the side-arms. A

discussion of the prevalence of thiophene as an electron-donating component in small-

molecule (Figure E1) and polymer hole-transporting materials (Figure E2) in PSCs is

provided in Appendix E.

Herein, a low-cost TPA core is used to build a series of hole-transporting materials,

each with a different side-arm composition. The effect of both molecular composition and

film-forming properties on the performance TPA-based HTMs in perovskite solar cells is

investigated. By engineering side-arms with different functionalities (i.e., a different

combination of electron-donating (donor) and electron-withdrawing (acceptor) units), it is

possible to tune the electronic properties. Therefore, the HOMO can be adjusted to a

suitable level conducive to hole-extraction from the perovskite active layer.

47

4.2. Materials and Methods

4.2.1. Materials

The molecular structures of all organic materials used in this study are given in

Figure 4.2. The molecular formula and molecular weight for each of the materials are

provided in Table 4.1. The first hole-transporting material is 5,5',5''-(5,5',5''-

(nitrilotris(benzene-4,1-diyl))tris(2-hexylthiophene), designated as “HTM01”. HTM01 is

the simplest and smallest molecule in the series with a TPA core flanked by a thiophene

unit for the purpose of extending π-conjugation. The hexyl chains increase solubility in

organic solvents. This compound has been previously synthesized by others via lithiation

and transmetallation.73 In Welch lab, Arthur Hendsbee successfully synthesized HTM01

via direct arylation as an alternative and facile procedure.

The second compound in this study is 5,5',5''-(5,5',5''-(nitrilotris(benzene-4,1-

diyl))tris(furan-5,2-diyl))tris(2-octylisoindoline-1,3-dione), or “HTM02”. HTM02 (also

synthesized by Arthur Hendsbee) has two structural differences compared to HTM01: i)

replacement of the thiophene unit with a furan group and ii) addition of an electron-

deficient phthalimide acceptor group. The furyl units were selected for two reasons. First,

it is considered to be a sustainable and “green” alternative to thiophene as a building block

in functional materials since it is derived from biological feedstocks.74–76 Second,

substitution of furan for thiophene as donor units in organic semiconductors can result in

an increase in hole mobility, as demonstrated by recent work in the Welch and Hill labs77

and by others.78,79 The phthalimide groups are electron-deficient and stabilize the frontier

energy levels of the molecule HTM02 relative to HTM01, resulting in a decrease in the

LUMO energy and a corresponding decrease in the bandgap energy.

48

The third compound is named HTM03. HTM03 is largest, most complex molecule.

Similar to HTM02, HTM03 has furan-phthalimide D-A units. A diketopyrrollopyrrole

(DPP) unit in the center of the side-arm was inserted to stabilize the frontier energy levels,

particularly the LUMO, to reduce the bandgap energy relative to HTM02. Large branched

alkyl chains (i.e., 2-ethylhexyl) help to disrupt strong π-π intermolecular interactions

between DPP units,80 which improves solubility. 1-ethylpropyl or “swallowtail” end caps

were used in place of the hexyl chains present in HTM01 and HTM02.

The fourth hole-transporting material is 7,7’-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-

b:4,5-b’]dithiophene-2,6-diyl)bis(6-fluoro-4-(5’-hexyl-[2,2’-bithiophen]-5-yl)benzo[c]-

[1,2,5]thiadiazole), which is also known as p-DTS(FBTTh2)2, but is labelled as “HTM04”

throughout this chapter. This material demonstrates high hole-mobility81 and has similar

HOMO energy levels as HTM01, HTM02 and HTM03. For this reason, it is appropriate

for use as a hole-transporting material in this study. HTM04 is also well-known as a high-

performance two-dimensional (2D) donor material in small-molecule organic solar cells.82

49

Figure 4.2. Molecular structures of the five HTMs investigated in this study. The TPA

cores are coloured blue and thiophene units are coloured red.

Table 4.1. The chemical formula and molecular weight of each HTM used in the study.

HTM Chemical Formula Molecular Weight

(g/mol)

HTM01 C48H57NS3 744.17

HTM02 C72H66N4O9 1131.34

HTM03 C147H168N10O18 2263.01

HTM04 C64H72F2N4S8Si 1219.87

Spiro-OMeTAD C81H68N4O8 1225.45

4.2.2. Thin-film Formation via Spin Coating

To characterize each hole-transporting material individually, thin-films were spin-

cast on glass substrates. Precursor solutions were prepared by combining the weighed solid

compound and the measured volume of solvent (chlorobenzene) in a 4.0 mL Teflon-capped

vial with a stir bar. The materials were weighed using an analytical balance and the solvent

was measured using a 1000-μL micropipette. Solutions of spiro-OMeTAD, HTM01 and

50

HTM02 were stirred overnight at RT and solutions of HTM03 and HTM04 were stirred on

a hotplate set to 90 °C.

The 25 mm × 25 mm glass substrates were cut from microscope slides. The

substrates were cleaned prior to use by scrubbing the surface with a mixture of lab detergent

(Sparkleen®) and DI H2O and then rinsing thoroughly with DI H2O. The substrates were

then sequentially rinsed with acetone and ethanol prior to UV/ozone cleaning for 20 min.

Substrates were loaded onto the chuck of a Laurell spin-coater immediately after

UV/ozone treatment. The solutions of spiro-OMeTAD, HTM01 and HTM02 were cast at

RT whereas the solutions of HTM03 and HTM04 were hot-cast due to their lower

solubility. Each solution was retracted from the vial using a 1 mL syringe. Approximately

200 μL of solution was then dispensed onto the center of the substrate through a 0.45 μm

PTFE filter in one aliquot. The spin-coat program was then run using the parameters

specified in Table 4.2. The high solubility of spiro-OMeTAD in organic chlorinated

solvents permits casting from a solution that is higher in concentration than the TPA HTMs

and HTM04.

Table 4.2. HTM deposition parameters.

HTM

Concentration

in CB

(mg/mL)

Solution

Deposition

Temperature

(°C)

Spin Speed

(rpm)

Spin

Duration

(s)

Spiro-OMeTAD 80 25 4000 30

HTM01 25 25 1000 90

HTM02 25 25 1000 90

HTM03 25 70 1000 90

HTM04 25 70 1000 90

4.2.3. PSC Fabrication Procedure

All devices were made with the same architecture: FTO/TiO2

compact/CH3NH3PbI3/HTM/MoO3/Ag. The FTO glass substrates and TiO2 layers were

51

prepared as described in Chapter 3. The perovskite active layers were formed via the

optimal sequential spin-coating method also described in Chapter 3.

After the active layers were converted to perovskite, the devices were removed from

the glovebox. All HTMs were deposited onto the perovskite layer in air via spin-coating

from chlorobenzene solutions and filtered through 0.45 µm PTFE filters. No dopants were

used in the devices comparing the different hole-transporting materials. However, a

detailed discussion of the effect of dopants on HTM performance can be found in

Appendix F with a comparison of JV curves for devices made with doped and undoped

spiro-OMeTAD in Figure F1. Deposition parameters for each HTM are given in Table

4.2. The top metal contact was deposited by thermal evaporation at < 4.0×10-6 torr in a

custom-built bell-jar evaporation system. A 10-nm layer of molybdenum (VI) oxide

(MoO3) was first deposited followed by 75 nm of Ag. The purpose of using a MoO3-Ag

top contact was to replace the commonly-used Au contact and reduce the overall cost of

the system.83 The final active area of each device was 0.032 cm2.

After fabrication, the devices were promptly transferred into an Ar-filled glovebox,

which was the location for J-V testing and the site of post-testing storage. To account for

hysteresis in the J-V curves, each cell was scanned first from a forward bias (+ 2.0 V) to

short-circuit (0.0 V) and then in the opposite direction. Each scan was repeated with three

different scan delays (5 ms, 10 ms and 100 ms) at a sampling rate of 0.01 V/step.

4.3. Results and Discussion

4.3.1. Material Characterization

Figure 4.3 shows an energy level diagram of the components of the PSC devices.

HOMO and LUMO energy levels of the organic hole-transporting materials were

determined by cyclic voltammetry (CV plots shown in Figure 4.4). The onset of oxidation

52

for each of the HTMs was measured as the intersection of the baseline of the CV plot and

the line tangent to the first oxidation peak. These potential values were referenced to the

oxidation potential of ferrocene, which has been measured previously to be 4.80 eV, and

are listed in Table 4.3.39 Detailed methods for the CV experiments and calculations of the

energy levels can be found in Section 2.7. It was confirmed that the HOMO levels of all

five HTMs were similar (~5 eV) and at an appropriate level for hole-extraction at the

perovskite interface.

Energy levels of the other components are included in the diagram for comparison

and are based on literature reports. The valence band maximum and the conduction band

minimum of the perovskite (CH3NH3PbI3) were previously determined by others.17 The

conduction band minimum of −5.5 eV for an air-exposed film of MoO3 is representative of

n-type MoO3 charge-transport layers used in organic electronic devices.84 In perovskite

solar cells, MoO3 is used as an n-type material to assist charge transfer from the HTMs to

Ag, a high work-function electrode. Dense TiO2 acts as the electron-transport/hole-

blocking layer with a work function of −4.0 eV.85 The work function of fluorine-doped

tin-oxide (FTO), the electron-collecting electrode, is −4.5 eV based on measurements

conducted using X-ray photoelectron spectroscopy (XPS).86

53

Figure 4.3. Energy level diagram for all components in the PSCs. The HOMO and LUMO

levels for HTM01-04 were determined by CV, with coloured bars representing the

bandgap. Only the HOMO level was measured for spiro-OMeTAD. The CBM and VBM

for CH3NH3PbI3 were determined by others.7

Figure 4.4. a) Full cyclic voltammograms (CVs) for each of the HTMs used in the study.

b) A zoomed plot of the CVs for the HTMs, depicting the onset of oxidation for each of the

five HTMs. All CVs were acquired by scanning at a rate of 100 mV/s. The values of the

HOMO and LUMO levels were calculated by comparing the onset of oxidation and

reduction (respectively) to the normal hydrogen electrode (NHE), assuming that the

HOMO of Fc/Fc+ is equal to 4.80 eV.12 CVs for HTM01, HTM03 and HTM04 were

acquired by Arthur Hendsbee, Jetsuda Areephong and Seth McAfee, respectively.

54

UV-Vis absorption spectra for the five HTMs are shown in Figure 4.5, with a

summary of electronic parameters given in Table 4.3. Optical bandgap energies (Eg)

acquired from both solution and thin-film spectra are defined by the onset of absorption

(λonset), which was measured to be the wavelength at which the line tangent to the lowest-

energy peak and the baseline intersect. In all of the HTMs, the onset of absorption

wavelength for the thin-film spectra is longer (i.e., red-shifted) relative to that of the

solution spectra (λonset,solution). This change is attributed to solid-state ordering within the

thin films, which is known to enhance electron delocalization through the π-conjugated

backbone of organic small-molecule semiconductors.87 Spiro-OMeTAD and HTM01 have

similar absorption profiles with maximum film absorption (373 and 377 nm, respectively)

in the near-UV region. HTM02 has an intermediate optical bandgap, with maximum

absorption at 445 nm for the thin film. HTM03 and HTM04 have the smallest Eg with

onset of absorption at 715 and 759 nm, respectively (for thin films). The narrow bandgap

and corresponding low-energy absorption of HTM03 is attributed to the additional DPP

unit in the side-arm.

Figure 4.5. UV-Vis absorption spectra of HTMs acquired for a) solution samples with

CHCl3 as the solvent and b) thin-films on glass substrates.

55

Table 4.3. Summary of optical and electronic parameters of HTMs.

HTM Eox vs.

Fc (V)

EHOMO

(eV)

Eg,solution

(eV)

Eg,film

(eV)

λonset,solution

(nm)

λonset,film

(nm)

Spiro-OMeTAD 0.55 −5.09 2.98 2.96 416 419

HTM01 0.14 −4.94 3.02 2.95 411 420

HTM02 0.26 −5.09 2.43 2.38 510 520

HTM03 0.24 −5.04 1.81 1.73 685 715

HTM04 0.23 −5.03 1.86 1.63 665 759

4.3.2. Perovskite Solar Cell Fabrication and Testing

All devices fabricated without an HTL displayed evidence of shorts and produced

no photocurrent, suggesting that the perovskite layer itself has pinholes – a source of shunt

pathways. On the other hand, all devices with HTLs displayed diode-like J-V curves with

no evidence of shorts (see Figure 4.6). A summary of the device performance metrics for

the best and average cells is provided in Table 4.4. Furthermore, it may be filling-in deep

pinholes in the rough perovskite active layer and may be blocking shunt pathways.

Figure 4.6. J-V curves of the best-performing cells with different HTLs. Also shown is

an illustration of the configuration of the planar PSCs.

56

Table 4.4. A summary of performance metrics for the best cells (i.e., those with the highest

PCE) with different HTLs. The plots were obtained by scanning from +2.0 V to 0.0 V at

a rate of 10 ms per data point under an illumination of 1.5 G. The average metrics for five

sample devices (each with a different HTL) are also provided with standard deviations in

the brackets. The shunt and series resistances listed are for the best-performing devices

only.

HTL Voc (V) Jsc (mA

cm-2) PCE (%) FF

Shunt

Resistance

(Ω/cm2)

Series

Resistance

(Ω/cm2)

Spiro-

OMeTAD

Best 0.88 17.16 6.58 0.43 1.65×106 0.04

Average 0.85(0.04) 16.36(1.28) 5.57(0.67) 0.40(0.03) - -

HTM01 Best 0.97 8.84 3.51 0.41 2.49×102 0.04

Average 0.94(0.03) 7.96(0.62) 2.83(0.43) 0.38(0.02) - -

HTM02 Best 0.92 4.18 1.99 0.52 5.68×102 19.98

Average 0.88(0.08) 3.91(1.04) 1.61(0.43) 0.48(0.13) - -

HTM03 Best 0.85 3.40 0.89 0.31 3.44×105 8.47

Average 0.87(0.04) 3.34(0.36) 0.88(0.06) 0.30(0.01) - -

HTM04 Best 0.89 10.02 2.76 0.31 1.74×105 0.02

Average 0.80(0.08) 7.47(3.04) 1.79(0.94) 0.28(0.03) - -

HTM01 performed the best among the TPA hole-transporting materials and

displayed the highest Jsc and PCE. Since the Voc of all devices is similar, Jsc is the major

contributing parameter to the observed differences in PCE and overall performance.

Interestingly, the two HTMs with thiophene units (i.e., HTM01 and HTM04) displayed the

highest Jsc compared to the two HTMs with the furan-phthalimide units. This may suggest

that the thiophene unit plays a role in the hole-extraction from the perovskite or hole-

transport to the metal contact. Compared to spiro-OMeTAD, all four new HTMs resulted

in devices with lower photocurrents, corresponding to lower overall performance.

4.3.3. Hole-Transporting Layer Surface Morphology

Figure 4.7 shows AFM images of the hole-transporting layer surfaces acquired

after device fabrication and testing. An image of the bare perovskite layer is shown for

comparison. The bare perovskite film consists of micron-sized crystalline grains with large

57

height deviations that resemble “mountains” and “valleys”. Ideally, the HTM would

conform to the rough surface of the perovskite and provide uniform coverage across the

entire active layer. Such morphology would result in devices of consistent HTL thickness,

and therefore, consistent PV performance. It is more likely, however, that the HTM will

tend to planarize the films, partially filling in the low “valleys” while resulting in thinner

coverage on the “mountains”. Herein, AFM was used to assess the morphology of each

HTL in terms of grain size and overall surface roughness.

It is observed that Spiro-OMeTAD forms the smoothest, most planar film over the

underlying perovskite layer with the lowest value of root-mean square roughness (RRMS)

(see Table 4.5). The spiro-OMeTAD solution was also significantly more concentrated

than the others. This may explain how spiro-OMeTAD effectively fills-in gaps between

the large perovskite domains and forms a planar surface over the perovskite crystals. From

the roughness parameters listed in Table 4.5, the roughness of the HTL appears to decrease

from HTM01 to HTM02 to HTM03. Although error bars on the roughness parameters are

not provided, the uncertainty in the values is quite high; the area that was sampled (one or

two 5×5 µm scans per sample) is very small relative to the entire HTL surface.

58

Figure 4.7. 2D AFM images of HTL surfaces of the five hole-transporting materials under

study. An image of a neat perovskite film (i.e., no HTL) is shown for comparison.

Table 4.5. Roughness parameters of HTL films on perovskite films obtained from 2D

AFM images. The parameters were calculated over an area of 25 µm2.

No HTL

(Neat

perovskite)

Spiro-

OMeTAD HTM01 HTM02 HTM03 HTM04

Surface Area (µm²) 26 24 28 24 25 24

RRMS a (nm) 35 12 58 28 17 20

Ra b (nm) 28 10 40 21 13 14

Rmax c (nm) 246 86 445 228 132 170

a RRMS, the root mean square (RMS) average of height deviations relative to the mean image data plane. b

Ra, the average of the absolute values of the surface height deviations relative to the mean plane. c Rmax, the

vertical distance between the highest and lowest data points in the image.

4.4. Conclusions

A design strategy for organic hole-transporting materials has been demonstrated

with applications in PSCs using a TPA-core and highly modular side-arms. By keeping

the HOMO level constant among the HTMs, the effects of shape, size, and conjugation

length on thin-film morphology and device performance could be compared. Following

this approach, the molecular structures of the HTMs could be easily modified, such as the

59

exchange of electron-rich (e.g., thiophene and furan) side-arm functionalities. As a result,

electronic properties such as the HOMO level can be tuned to an appropriate value for hole

extraction from the perovskite. Furthermore, a trend was observed in the performance of

the TPA-based hole-transporting materials and the type of functionalities used in the side-

arms: devices with the HTM including a thiophene unit (HTM01) in the side-arm produced

higher photocurrent than those with HTMs containing the furan-phthalimide unit (HTM02

and HTM03). The bandgap energy of the HTM does not appear to have a significant

influence on PV performance since HTM02 and HTM03 display similar performance, yet

HTM03 has a significantly smaller bandgap. Comparison of the TPA-based donors to

HTM04 confirmed that another thiophene-containing molecule (albeit, linear in shape)

generated greater photocurrent than those with furan-phthalimide. Furthermore, HTM04

has a significantly narrower bandgap than HTM01 yet displays a similar Jsc in PSC devices.

It was shown that the surface roughness of the hole-transporting layer formed by

HTM01, HTM02, HTM03 and HTM04 was not significantly different and therefore was

not a major factor in the observed differences in PV performance. However, it was

observed that spiro-OMeTAD forms a smoother layer than the other four HTMs, which

may be attributed to the high solution concentration. The high solubility of spiro-OMeTAD

gives it the ability to better planarize the rough perovskite film and contributes to the high

PCE.

Development of simple TPA-based hole-transporting materials via facile synthetic

methods is commercially relevant. Having identified a unique structure-performance

relationship, further investigation of the effect of heterocycles in the sidearm units of such

HTMs may proceed. Future optimization in the molecular design of TPA-based HTMs

could focus on adjustments to the side-arm units including:

60

i. The use of sulfur-based heterocyclic functionalities;

ii. Exchanging R-groups (e.g., methoxy) to enhance solubility, affording the

preparation of solutions with higher concentrations, which would lead to

thicker HTL film-formation and better coverage of the perovskite surface.

4.5. Future Work

Two other important factors to consider in the development of alternative HTMs

are air and thermal stability. In order to be viable as a commercial material, the HTM must

be able to operate in humid and at elevated temperatures. For this reason, it is proposed

that the following comparative experiments be conducted for the series of HTMs:

i. Thermogravimetric analysis (TGA) to monitor physical changes to the

materials under heat exposure;

ii. Photovoltaic performance (e.g., J-V) measurements under controlled

atmospheric conditions (e.g., elevated humidity levels).

From this study, it was discovered that the molecular structure of the side-arms of

the TPA-based hole-transporting can influence performance in perovskite solar cells.

Modifications to HTM01 and HTM02 can be made to further investigate the effect of

thiophene, furan, phthalimide and combinations of the three. Figure 4.8 depicts the

molecular structures of two new HTMs. HTM01 is modified by replacing the thiophene

unit with furan, whereas the furan unit in HTM02 is replaced by thiophene while keeping

the phthalimide group present. It is proposed that by fabricating devices with four different

hole-transporting materials under the same, controlled processing conditions, insight into

the relationship of hole-transport in perovskite solar cells and the functional groups in the

HTM can be gathered. This methodology is advantageous with modular designs such as

61

HTM01 and HTM02 since new derivatives can be readily synthesized and screened for

device performance.

Figure 4.8. Proposed modifications to HTM01 and HTM02 for future studies in TPA-

based HTMs.

In a separate study discussed in Appendix G, non-conjugated polymers were

investigated as inert additives in hole-transporting layers of perovskite solar cells. It was

shown that addition of non-conjugated polymers can increase the performance of devices

(Figure G1 for J-V curves) by improving the planarity of the hole-transporting layer

formed by HTM04 (see Figure G2 for AFM images). This strategy is a proposed as a

viable technique to enhance device performance in perovskite solar cells and can be applied

to new TPA-based hole-transporting materials as well.

62

CHAPTER 5 – STAR-SHAPED DONOR MATERIALS FOR SMALL-

MOLECULE ORGANIC SOLAR CELLS

5.1. Background

5.1.1. Transition from Hole-transporting Materials to Donors

It was shown in Chapter 4 that the molecular composition and film-forming properties

of the TPA-based hole-transporting materials can influence device performance in PSCs.

Ultimately, the select TPA-based compounds were not ideal candidates for HTMs – all

three displayed significantly lower performance than spiro-OMeTAD in perovskite solar

cells. However, these materials might be better suited for other photovoltaic applications.

The three newly designed small molecules used as HTMs in PSCs all have appropriate

energy levels and absorption profiles to be considered for use as donor components in

organic solar cells. In some instances, such compounds can serve in both PSC and OSC

devices. For example, in a recent paper by Cheng et al. a single organic compound was

used for the dual-purpose as a hole-transporting material in a perovskite solar cell and as a

donor in a bulk heterojunction organic solar cell.88 Similarly, HTM04 (aka p-

DTS(FBTTh2)2) is a high-performance donor material in OSCs,82,89 but it was shown in

Chapter 4 to also function as an HTM in PSCs.

5.1.2. Selection of a Complementary Acceptor

It is of interest to develop fullerene-free organic solar cells. In small-molecule

organic photovoltaic systems, fullerene derivatives, such as phenyl-C61-butyric acid methyl

ester (PC61BM or PCBM), are the gold-standard electron-acceptors. However, fullerenes

are expensive, difficult to synthesize and display weak absorption of visible light;90,91

factors that are disadvantageous in OSCs. The goal herein is to investigate the new TPA

small molecules as donors with custom-built non-fullerene acceptors.

63

Fullerenes, such as PCBM, have a spherical three-dimensional (3D) shape and a 3D

π-conjugated pathway (see Figure 5.1). As result, charge transport in fullerenes is

isotropic;92 charge carriers are transported in all directions through the fullerene. The

success of fullerenes as acceptors in organic solar cells is largely attributed to its shape,

which allows for high electron mobility in three dimensions.93 In fact, the high-

dimensionality and 3D shape is a sought-after trait for the design of non-fullerene acceptors

(see Figure H1 in Appendix H for examples of 3D non-fullerene acceptors).

In contrast, planar organic molecules are two-dimensional (2D) in shape, yet have

a π-conjugated pathway that runs in one plane (i.e., a 1D π-conjugated pathway along the

length of the molecule),92 A well-known example of an organic molecule with a 2D shape

is HTM04, aka “p-DTS(FBTTh2)2” (Figure 5.1).82

Figure 5.1. An illustration of the concept of opposing molecular geometry in donor-

acceptor BHJ OPVs.

64

Some of the best-performing small-molecule OSCs consist of a 3D fullerene

acceptor with a planar 2D donor.94 For clarity, these types of systems will be referred to as

“2D-3D” systems herein (indicated by a red arrow in Figure 5.1). The dissimilarity in

shape between the two complementary materials is thought to drive self-assembly and

produce a solid-state morphology in bulk heterojunction active layer that is conducive to

efficient charge transfer, separation and extraction. These pathways are essential for charge

transportation and collection at the electrodes. Similar to PCBM, TPA-based compounds

also demonstrate high dimensionality and assume a 3D shape. Evidence for the sp3-

hybridization of the N-atom at the TPA core in other related organic semiconductors has

been demonstrated by others using density functional theory (DFT) calculations64 and is

suggested to be the result of steric interactions between the side-arm units.92 As a result,

the molecule assumes a 3D propeller-shape with π-conjugated pathways extending in three

dimensions. To date, all TPA donor materials reported in the literature for use in OSCs are

accompanied with a fullerene acceptor (e.g., C60, PC61BM, PC71BM, etc.), While there

have been some examples in the literature of 3D TPA donors in OSCs achieving modest

PCEs between 3%95–97 and 4%,98 the issue of the use of fullerene acceptors remains

unaddressed.

5.1.3. Custom-Made D-A Pairs

Developing fullerene-free OSCs requires a D-A pair with the following criteria:

i) Distinct geometry/topology to ensure phase separation, allowing for the

formation of homogeneous percolation pathways within the active layer and

charge transport to the electrodes,

ii) Complementary light absorption to maximize photon-harvesting,

65

iii) HOMO/LUMO energy levels that are offset to ensure efficient electron

transfer (LUMO of donor to LUMO of acceptor and HOMO of donor to

HOMO of acceptor),

iv) Thermal and air stability,

v) Low cost with a low embodied energy,

vi) Photo-stability.

The goal herein is to utilize the TPA-based hole-transporting materials developed in

the previous chapter as donor molecules in fullerene-free organic solar cells. It is now

reported how the molecular structure of such compounds will affect the electronic,

structural and morphological properties in thin-film D-A systems. In this case, 3D trigonal

pyramidal conjugated molecules, such as those with a TPA core, are ideally paired with a

planar 2D acceptor to form a “3D-2D” system and satisfy the first criterion (indicated by

the blue arrow in Figure 5.1). Although the geometries of these compounds are agreeable,

the absorption, electronic properties and thin-film morphology must be characterized to

ensure that the other criteria are met.

5.2. Materials and Methods

5.2.1. Materials

Molecules D1, D2 and A1 were synthesized in the Welch lab by Arthur Hendsbee.

The structures of the compounds are depicted in Figure 5.2. The 3D, TPA-based donor

compounds are named “D1” and “D2” for simplicity. The reader is reminded that “D2” is

named “HTM02” in Chapter 4. D1 and D2 differ only in the terminal alkyl chains; D1

has bulky 1-ethylpropyl (aka “swallowtail”) groups while D2 has linear hexyl chains.

A linear, 2D acceptor with a DPP core was selected as the acceptor. This

compound, which is named “A1” for the purpose of this chapter, has been previously

66

reported to have favourable optoelectronic properties for electron transport.99 A1 contains

two bridging thiophene units that connect the DPP core to the electron-deficient

phthalimide end-caps. The terminal and core alkyl groups are swallowtail and octyl chains,

respectively. The design of this compound allows for the substitution of a variety of R-

groups that can influence molecular interactions and self-assembly in the solid-state (vide

infra).

In order to purify the organic compounds used in this work, standard organic

purification techniques were applied by Arthur Hendsbee, including organic extractions,

column chromatography, filtrations and recrystallizations. 1H and 13C nuclear magnetic

resonance (NMR) spectroscopy and mass spectrometry were used to confirm the identity

of the molecules as well as to detect organic impurities. In addition, elemental analysis was

used to confirm the absence of impurities such as inorganic compounds (e.g., SiO2, which

is a potential contaminant during chromatography) that are not visible using other methods.

The samples of each material used herein were qualitatively deemed to be pure when the

analytical methods listed above detected no extraneous signals. It is recognized that this

method of purity assessment is dependent on the detection limit of each instrument.

Although each of the analyses provides a qualitative assessment of sample purity,

quantitative measurements of purity were not conducted.

67

Figure 5.2. Molecular structures of D1, D2 and A1.

5.2.2. Thin-film Formation via Spin Coating

All thin-films were formed via spin-coating from solution. Unless otherwise stated,

single-component thin-films were spun from solutions using CHCl3 as the solvent with a

concentration of 10 mg/mL. D1:A1 and D2:A1 blend-film solutions used CHCl3 as the

solvent with a total solid concentration of either 30 or 20 mg/mL (e.g., 10 mg of D1 and 10

mg of A1 per 1 mL for a 20 mg/mL blend solution). Neat solutions (i.e., no solvent

additives) were prepared by combining the weighed (using an analytical balance) solid

compound(s) and the measured volume (using a micropipette) of CHCl3 in a 4.0 mL Teflon-

capped vial with a stir bar. Solutions were gently heated for 10 min under stirring and then

allowed to stir overnight at room-temperature to dissolve the materials.

68

The films were spin-cast using a spin speed of 1000 rpm for a duration of 90 s. A

description of the spin-coating procedure for organic thin-films can be found in Section

4.2.2.

5.2.3. Organic Solar Cell Fabrication Procedure

OSC devices were fabricated on 25 mm × 25 mm pre-patterned ITO-coated glass

substrates. Substrates were cleaned by gently scrubbing the surface with a solution of DI

H2O and Sparkleen® lab detergent using a soft toothbrush. The substrates were then

sonicated vertically in a solution of DI H2O and Sparkleen® for 15 min after which they

were rinsed thoroughly with DI H2O, dried under a stream of filtered air, and then sonicated

vertically in acetone for 15 min. The process was then repeated, but using ethanol as the

solvent. The substrates were then treated under a UV/ozone lamp for 20 min.

Immediately following UV/ozone exposure, a layer of PEDOT:PSS was deposited

as the HTL from a chilled aqueous colloidal suspension purchased from Heraeus (Product:

CleviosTM AI 4083). The substrate was mounted onto the chuck of the spin-coater and

approximately 0.3 mL of the dispersion was dispensed onto the surface through a 0.45 μm

PVDF filter before spinning at 2000 rpm for 45 s. The film was removed at the edges of

the substrate using a lint-free Texwipe® dampened with DI H2O to expose the ITO

contacts. The PEDOT:PSS film was then annealed on a hotplate with a surface temperature

of 140 °C for 10 min. Annealing the PEDOT:PSS film at such temperature has been shown

by others to increase its conductivity, resulting in better-performing devices.100

After cooling the substrates to room-temperature, the active layer was deposited by

spin-coating the surface using blend solutions. The total solids concentration of the

solutions was 20 mg/mL with a 1:1 weight ratio of donor to acceptor, using CHCl3 as the

solvent. For each active layer, 0.2 mL of solution was filtered directly onto the substrate

69

through a 0.45 μm PTFE syringe filter followed by spinning at 1000 rpm for 90 s. The ITO

contacts were then exposed by scraping away the blend film using a razor blade. Once all

active layers were spun, the substrates were promptly cycled into the glovebox and

mounted into a substrate holder with a metal shadow mask.

The top metal contact was deposited using an in-house designed thermal evaporator

equipped with two evaporation sources. Calcium was loaded into an alumina crucible as

the first source and aluminum pieces were secured to a tungsten filament as the second

source. Once the substrates were transferred into the evaporation chamber, the system was

pumped down to <1×10-6 torr. The metal cathode was formed by sequential thermal

evaporation of 7.5 nm of Ca and 100 nm of Al. The chamber was allowed to cool for 30

min under vacuum before the substrates were transferred directly into the glovebox where

they were promptly tested.

5.2.4. Organic Solar Cell Testing

Device testing was performed using a Keithley 236 Source-Measure Unit in an Ar-

filled glovebox under 100 (±1) mW/cm2 illumination (AM 1.5G). J-V measurements were

collected for each of the 18 devices on each substrate by scanning first from 1.2 V to -0.2V,

1.2 V to -6.2 V and finally from 1.2 V to -0.2 V, at a rate of 0.05 V/step with a delay of 5

ms between steps.

5.3. Results and Discussion, Part I

5.3.1. Materials Characterization, Part I

The first step in the study was to select a suitable solvent for the preparation of thin-

film precursor solutions. Ideally, solutions with high concentrations are preferred for spin-

coating to ensure uniform coverage of the substrate and adequate film thickness (i.e., 100

nm). For this reason, it is favourable to use solvents in which the active materials are highly

70

soluble. The approximate solubility of each of the three materials in three different solvents

with varying polarity (in order of decreasing polarity: ethyl acetate, 2-methyl

tetrahydrofuran, and chloroform (CHCl3)) were determined by an undergraduate honours

student, Liz Kitching. The solubilities were quantified from a calibration curve constructed

using solution UV-Vis data. All three of the materials displayed the highest solubility in

CHCl3. For this reason, all subsequent solutions in this study were prepared using CHCl3

as the solvent.

UV-Vis absorption spectra for the three materials are plotted in Figure 5.3 a). The

absorption profiles of D1 and D2 are almost identical. This can be explained by the

structural similarity of the two materials; both contain the same light-absorbing π-

conjugated backbone. The absorption of A1 extends to longer wavelengths than both of

the donor materials and is reflective of its narrow bandgap. This can be explained by the

presence of the DPP chromophore; electrons are localized in low-lying molecular orbitals

at the DPP core,101 and low-energy photons are sufficient to induce electronic transitions.

By combining either D1 or D2 with A1, the materials absorb across a broad range of the

visible-light spectrum, fulfilling the second criteria for a D-A pair: complementary light

absorption.

The CV plots for the three materials are shown in Figure 5.3. As expected, D1 and

D2 exhibit very similar CVs, owing to their similar molecular structures. The oxidation

potentials of both donor compounds are well-defined by the sharp oxidation peaks,

allowing accurate elucidation of the HOMO levels. The bandgap energies (Eg,CV) for each

of the materials are listed in Figure 5.3 d) and were obtained by taking the difference

between the calculated HOMO and LUMO energy levels. These values are in good

agreement with the values of the bandgap (Eg,onset) that were measured from the UV-Vis

71

onsets of absorption (λonset). A description of how Eg,onset is calculated from λonset is

provided in Section 2.5. A1 has a high oxidation potential, corresponding to a deep HOMO

level. The energy offset between the LUMO levels of D1 and A1 and D2 and A1 are 0.53

eV and 0.48 eV, respectively. In order for charge separation to occur, the offset must be

larger than the exciton binding energy. Comparison of the energy offsets to the binding

energies of other organic small molecules,30 suggests that the offsets may be sufficient for

charge separation. Therefore, the third criterion for a D-A pair is fulfilled: appropriate

energy level alignment.

Figure 5.3. a) UV-Vis absorption spectra for solution samples of D1, D2 and A1 using

CHCl3 as the solvent. b) CV plots for D1, D2 and A1. c) An energy-level diagram for D1,

D2 and A1, showing the HOMO and LUMO energy levels. d) A table summarizing the

optoelectronic properties of the three materials, where Eg,CV and Eg,optical are the bandgap

energies determined by CV and UV-Vis, respectively, and λonset is the wavelength at the

onset of absorption, that is, the longest wavelength absorbed by the material.

72

5.3.2. Thin-film Characterization, Part I

The D1-A1 and D2-A1 systems meet the criteria for a suitable D-A pair and can be

investigated in thin film systems. It is important to understand the properties of each

material in independent systems before blend films are made. Neat films of D1, D2 and

A1 were spin-cast on cleaned glass substrates for the purpose of solid-state thin-film

characterization. Each film was thermally annealed at incrementally higher temperatures

for a duration of 10 minutes in a procedure referred to as “sequential thermal annealing.”

Thermal annealing is a common processing technique applied to thin-films in OPV

applications to induce morphological changes and dramatic increases in PCE.102–104

Furthermore, it is important to understand how an organic material responds to heat in a

thin film solar cell; under normal operation in full-sun exposure, the temperature of a solar

cell will inevitably rise.

Figure 5.4 depicts UV-Vis absorption spectra for neat films of each of the three

materials before and after sequential thermal annealing. The absorption spectra of D1 and

D2 exhibit only slight changes upon thermal annealing; a slight blue-shift in the low-energy

band of D1 and an overall decrease in the intensity of the absorption of both D1and D2 is

observed with increasing annealing temperature. In contrast, A1 exhibits significant

changes in the band structure after annealing, suggesting that electronic changes occur

within the material. Most notably, three bands merge in the red-region of the spectrum

(between 550 and 700 nm).

73

Figure 5.4. UV-Vis absorption spectra acquired by Liz Kitching for neat films of a) D1,

b) D2 and c) A1 during the sequential thermal annealing experiment. The absorption

spectrum was first acquired for each as-cast film. Each film was then annealed by placing

the substrate onto a hotplate heated at 100 °C for 10 min after which a second absorption

spectrum was acquired. This process was repeated, sequentially annealing at incrementally

higher temperatures.

Thin-film blends of D1 and A1 and D2 and A1 were spin-cast on glass substrates

by Liz Kitching. Similar to the one-component films, sequential thermal annealing was

applied to the blend films and UV-Vis absorption spectra acquired after each annealing

step. The absorption spectra of the blend films are depicted in Figure 5.5.

Figure 5.5. UV-Vis absorption spectra acquired for blend films of a) D1-A1 and b) D2-

A1 during the sequential thermal annealing experiment. Solutions for both blends were

prepared using the donor and acceptor compounds in 1-to-1 weight ratios.

74

The spectra of both unannealed (i.e., as-cast) blend films reveal two distinct bands

in the region between 550 and 700 nm that are absent in the A1 as-cast neat film (see Figure

5.3 c)). The resolution of these bands suggests that the presence of the donor influences

the solid-state ordering of A1 in the thin-film blend. Similar to the phenomenon observed

in the neat film of A1, thermal annealing results in a change in the band structure of A1

within both blend films; three bands emerge upon annealing. From the UV-Vis spectra in

Figure 5.4, it is clear that the absorption of the donor and acceptor is unbalanced in the

blend films; the magnitude of absorption of D1 and D2 is considerably greater than that of

A1. For this reason, blends with higher quantities of A1 relative to the donor were

investigated. UV-Vis spectra (Figure I1) and XRD powder patterns (Figure I2) for films

with varying ratios of donor and acceptor are provided in Appendix I.

The interactions between the donors and A1 within the blend films were further

probed using photoluminescence (PL) spectroscopy (Figure 5.6). In this experiment, the

thin film samples were irradiated with monochromatic light of a wavelength, λex. A

photodetector collected the emitted light across a range of wavelengths (λem), providing

evidence for fluorescence. PL spectra of D1 and D2 neat films were first acquired using

an excitation wavelength of 443 nm and collecting emitted radiation from 500 to 900 nm.

The A1 neat film displayed minimal fluorescence when excited at the wavelength of

maximum absorbance (i.e., 587 nm). For this reason, the PL spectrum of A1 is not shown.

75

Figure 5.6. PL spectra comparing the fluorescence of one-component donor films with

two-component donor-acceptor (1:1 weight ratio) blend films.

As illustrated in Figure 5.6 a) and b), both D1 and D2 exhibit significant

fluorescence in single-component samples. That is, the electrons are effectively excited

from the LUMO to the HOMO and subsequently relax to the ground state, emitting a photon

in the process. However, the blend films with equal quantities of donor and acceptor

display negligible fluorescence. This indicates that the exciton dissociates at the D-A

interface, separating into free charge carriers. The electrons are effectively transferred from

D1 or D2 to A1, and are provided with alternate relaxation pathways. As a result, electrons

do not relax from the LUMO to the HOMO of the donor and the intensity of the

fluorescence is reduced or “quenched”. PL quenching provides additional evidence for D-

A interactions within the blends, which is a favourable trait for materials in organic solar

cells.

Thus far, all experiments described were conducted on both D1-A1 and D2-A1

systems in parallel. For the purpose of streamlining the optimization of thin-films for

application in organic solar cells, one system is subsequently studied in greater depth.

Since D1 and A1 have identical alkyl end-caps (i.e., swallowtail), which suggests molecular

compatibility, the D1-A1 system was selected for further optimization.

76

It is important to consider the crystallinity of the materials within the active layer;

formation of well-ordered crystalline domain can enhance charge transport.105 The changes

in the absorption profiles of the blend films induced by thermal annealing (see Figure 5.5)

are indicative of electronic changes that may be related to the crystallinity or solid-state

ordering of the materials. To further probe the structure of the films, XRD experiments

were conducted. XRD patterns of thin films of D1 and A1 are shown in Figure 5.7 a) and

b), respectively, and are compared to a two-component blend film (1:1 weight ratio of D1

and A1, Figure 5.7 c)) before and after annealing at an intermediate temperature (150 °C).

Figure 5.7. XRD patterns of one-component thin-films of a) D1 and b) A1 and c) a two-

component D1-A1 blend film acquired before and after thermal annealing for 120 min at

150 °C. The scans were acquired with a range between 3° and 20°, however, no diffraction

peaks were observed above 2θ = 10°. Thus, only narrow range is used to clearly the show

the peaks.

Based on the absence of diffraction peaks for any of the as-cast films, it is assumed

that the materials are amorphous within the films or consist of sub-nanometre grains. Upon

annealing, the pattern of D1 does not change; no diffraction peaks are observed in the as-

cast nor the annealed film, indicating that the material remains amorphous. However, a

single diffraction peak is observed for the A1 film at 2θ = 4.3° after annealing – an

indication of solid-state ordering. After annealing the blend film, a peak emerges at the

same position as in the A1 film, indicative of crystallization of A1 domains.

77

In a separate experiment, the effect of annealing temperature on crystallinity of the

blends is illustrated by comparing XRD patterns of films annealed at 150 °C and at 180 °C

(see Figure 5.8). Annealing at a higher temperature results in a diffraction peak at the same

angle as the film annealed at 150 °C and of the annealed A1 film. This suggests that A1

assumes the same crystalline phase regardless of the annealing temperature. However, the

film annealed at high temperature displays a peak of higher-intensity; an indication of the

growth of larger crystallites.

Figure 5.8. XRD patterns of separate films of D1-A1 blends (1:1 weight ratio) comparing

the effect of annealing temperature on crystallinity. The films were annealed for 30 minutes

at either 150 or 180 °C.

5.3.3. Organic Solar Cell Testing, Part I

Based on the differences in the absorption spectra (Figure 5.5) and XRD patterns

(Figure 5.7 and 5.8) of the blend films, it is clear that electronic and structural changes

occur upon thermal annealing. To determine the effect of thermal annealing on PV

performance, OSCs were fabricated on three separate substrates using D1:A1 active layers

employing a conventional device architecture (see Figure 5.9 b)). A detailed fabrication

procedure can be found in Section 5.2.5. Before deposition of the metal cathode, each

78

substrate was annealed on a hotplate in the glovebox at a different temperature (or tested

without annealing).

All devices displayed linear J-V curves, with very high series resistance and very low

Jsc. Regardless of annealing condition, very little photocurrent was generated suggesting

that the system was not operational in OSC devices.

Figure 5.9. a) J-V curves of representative devices with D1-A1 active-layer blends in a

1:1 ratio. b) An illustration of the layered “conventional”-style architecture. c) A table of

performance metrics. d) Photographs of the completed devices.

To gain insight into the cause of the poor device performance, AFM images of the

active layers were acquired after device testing (Figure 5.10). The images reveal the

topography of the active layer surfaces and allow for quantification of the domain sizes.

The as-cast film consists of very small domains with minimal phase-separation. Annealing

at 150 °C induces growth of larger features that are likely crystalline domains of A1 which

correspond to the diffraction peak observed in the XRD pattern. The film annealed at 180

°C consists of large columnar crystallites up to a micron in breadth. These images correlate

79

well with the XRD patterns and confirm the growth of larger crystallites upon high-

temperature annealing. The large features observed in the images of the annealed films are

likely crystalline domains of A1.

Figure 5.10. 2D AFM images of the D1:A1 active-layer surfaces of the OSC devices.

5.3.4. Conclusions, Part I

The binary blend of D1 and A1 was determined to be a poor system for use in

organic solar cells. Despite evidence for PL quenching (Figure 5.6), and therefore

molecular compatibility as a D-A pair, preliminary devices performed poorly.

AFM reveals very different morphologies between as-cast and annealed films, none

of which are likely to display good performance as the active layer in OSCs. Based on the

AFM and XRD data, it can be concluded that: i) the amorphous nature of the as-cast films

consists of few charge-transport or percolation pathways and ii) the over-crystallization of

A1 upon annealing forms domains that are too large for efficient charge generation.

For this system, control of the D-A film morphology was used as a strategy to

improve the PV performance. Two strategies that can be used to influence the film

80

morphology are: i) solution-processing techniques and ii) molecular engineering. Two

examples of solution-processing techniques are the addition of solvent additives and

solvent-vapour annealing (SVA). In a preliminary study described in Appendix J, it was

observed that addition of solvent additives (Figure J1) and solvent-vapour annealing

(Figures J2 and J3) modifies the absorption properties of the materials. However, the

focus of the project herein is to apply a molecular design strategy to tune the self-assembly

of the acceptor and enhance the morphology of the thin films.

5.4. Results and Discussion, Part II

5.4.1. New Acceptors: A Molecular Design Strategy

Moving forward, the unfavourable over-crystallization of the acceptor was

addressed through molecular design. The molecular structure of the 2D acceptor was

modified to influence its self-assembly within the thin film. D1 continued to be used as the

donor in the system for its 3D shape and amorphous nature.

One of the simplest approaches to changing the self-assembly and crystallinity of

A1 is by modifying the alkyl chains, taking advantage of the facile synthetic procedure and

infinite tunability of this molecular framework. By substituting different alkyl chains for

the octyl and swallowtail groups at the central and terminal positions of A1, respectively,

the solid-state self-assembly can be controlled to change the morphology of the resulting

thin films. Following this approach, another graduate student researcher in the Welch lab,

Arthur Hendsbee, synthesized a series of derivatives of A1 (Figure 5.11), each with

different functional units and/or alkyl chains (Table 5.1). Each derivative was designed to

be less crystalline than A1 to avoid the large-domain phase separation observed in the D1-

A1 system (recall Figure 5.10). Since the major driving force for crystallization in π-

81

conjugated materials is π-π interactions between adjacent molecules, side chains were

selected to disrupt the π-π interactions and reduce the tendency to crystallize.

Figure 5.11. Molecular structures of A1 and the four new acceptor derivatives.

Table 5.1. Molecular components used in the acceptor derivatives.

Central R-groups Terminal R-group End-Cap Unit

A1 Octyl 1-Ethylpropyl Phthalimide

A2 2-Ethylhexyl 1-Ethylpropyl Phthalimide

A3 2-Ethylhexyl Octyl Phthalimide

A4 Octyl Octyl Napthalimide

A5 2-Hexyldecyl Octyl Phthalimide

Similar to A1, A2 has terminal swallowtail R-groups, but has a branched 2-

ethylhexyl chain stemming from the DPP core as opposed to the linear octyl chains on A1.

A3 differs from A2 with the substitution of the swallowtail groups for octyl chains. The

bulky branched chains at the core are expected to increase the spatial separation of the DPP

units of adjacent molecules thereby weakening the π-π interactions and reducing the

tendency to crystallize. In comparison, the linear octyl chains of A1 display strong van der

Waals forces, which drive the molecules into a stacking crystalline arrangement. Similar

to A3, A5 has terminal octyl chains, but larger branching central groups (2-hexyldecyl).

82

A4 is unique with larger naphthalimide groups as the terminal electron-withdrawing units

in place of phthalimide. Compared to phthalimide, naphthalimide groups induce a greater

torsional strain within the π-conjugated backbone, resulting in a less-planar molecule and

weaker π-π interactions.

5.4.2. Materials Characterization, Part II

To determine if each of the new acceptors is electronically-compatible with D1, CV

experiments were conducted and the HOMO and LUMO energy levels were calculated

based on the oxidation and reduction potentials, respectively. A description of how the

energy levels were calculated is provided in Section 4.3.1. The energy level diagram in

Figure 5.12 a) shows the relative positions of the frontier molecular orbitals compared to

those of D1. Normalized CV plots are provided in Figure 5.12 b). The similar energy

levels of the all five acceptors can be explained by the similar conjugated backbone

structure.

Figure 5.12. a) Energy levels of the frontier molecular orbitals of D1 and the five acceptors

determined by CV. The coloured bars represent the bandgaps of each material. b) CV

plots for the acceptor derivatives, normalized such that the current of the first oxidation

peak is set to unity. The onsets of oxidation and onset of reduction for each material were

used to determine the HOMO and LUMO energy levels, respectively. Each CV was

acquired by scanning at a rate of 100 mV/s.

83

5.4.3. Thin-film Characterization, Part II

Thermal annealing was applied to thin-film blends of D1-Ax systems as a

processing technique. The high-temperature condition (i.e., 180 °C) was selected to gain

insight into the robustness of the system against crystallization. UV-Vis spectra were

acquired on two separate films of each blend: i) one unannealed film and ii) a different film

annealed at 180 °C for 10 min after spin-casting. All films were spin-cast onto glass

substrates from CHCl3 solutions at concentrations of 20 mg/mL with a donor:acceptor

weight ratio of 1:1 (D1:Ax).

The UV-Vis absorption spectra are compared in the left-hand column of Figure

5.13. For all unannealed/as-cast blend films, the absorption of D1 (350-500 nm) is

considerably more intense than the absorption attributed to the acceptor (550-700 nm).

Significant changes in the absorption profiles of the blends with A2 and A3 are observed

after annealing (Figure 5.13 b) and c), respectively). For the D1-A2 blend, three notable

changes occur with annealing. First, the two high-energy bands attributed to D1 are blue

shifted, similar to what is observed in the D1-A1 blend. Second, a third band is observed

in the A2 region, increasing the overall breadth of the absorption. Third, there is an overall

relative increase in intensity of the low-energy absorption.

For the D1-A3 film, there is a large red-shift in the low-energy (A3) absorption

upon annealing. The two bands also become more distinct with a greater energy-separation.

Like A2, a third low-intensity absorption band emerges at the high-energy (blue) end of

absorption (580 nm). Also worthy of note is a blue-shift in the low-energy band of D1

from 445 to 425 nm.

84

In contrast, the blend with A4 shows no change in absorption when annealed. The

absorption of A4 is characterized by a single broad band and absorbs weakly compared to

the other five acceptors in blends with D1.

For the D1-A5 system, only slight differences are observed in the absorption of the

annealed film versus the as-cast film. A new shoulder in the A5 absorption is observed at

low-energy, which increases the onset of absorption from 695 to 715 nm. No change is

observed in the high-energy region of the spectrum, indicating that the absorption of D1 is

not influenced by annealing conditions in this system.

The XRD powder patterns for all of the blend films are also shown in Figure 5.13

(g-k) in the right-hand column. Similar to the D1-A1 system, all other blend films display

no diffraction peaks before annealing. When annealed, the blends with A1, A2 and A3

display a single diffraction peak at angles of 4.25, 5.75 and 6.15°, respectively. Although

a value for d can be determined based on the XRD patterns using Bragg’s Law, no crystal

structure data for these compounds were obtained (attempts were made to grow single

crystals, but yielded no crystals with large enough dimensions for single-crystal XRD

experiments). For this reason, it cannot be concluded whether the peak in each of the three

powder patterns corresponds to π-π or alkyl-alkyl spacing.

Even after annealing at high temperature, no peaks are observed in the D1-A4 blend

film. These patterns are in agreement with UV-Vis spectra, suggesting that neither

electronic nor structural properties of either material in the blend change after annealing.

A single broadened peak can be distinguished from the noise in the post-annealed D1-A5

film. The breadth and low-intensity of the peak suggests that crystalline domains form

upon annealing, but are of very small dimensions.

85

Figure 5.13. Thin-film characterization of D1:Ax blended films (1:1 by-weight) as-cast

(dashed lines) and after thermal annealing at 180 °C for 10 min (solid lines). UV-Vis

absorption spectra (a-e) and XRD patterns (g-k) are shown in the left- and right-hand

columns, respectively. The scans were acquired with a range between 3° and 20°, however,

no diffraction peaks were observed above 2θ = 10°. Thus, only a narrow range is depicted.

86

The AFM images reveal the morphology of the thin-film blends and are shown in

Figure 5.14. When unannealed, all blend films display a featureless morphology with no

indication of phase separation; this is in agreement with the lack of diffraction peaks in the

XRD patterns. Similar to the D1-A1 blend (shown again in Figure 5.14 for comparison),

the D1-A2 and D1-A3 blends develop large crystalline domains upon annealing. The D1-

A2 annealed blend consists of large mountainous domains measuring microns in lateral

dimension, with height deviations up to 200 nm. The D1-A3 blend has smaller, more-

compact pebble-like features.

The annealed film of the D1-A4 blend becomes more textured after annealing;

however, the features are very small. Based on the lack of diffraction peaks in the XRD

pattern (Figure 5.13 j)) these features are likely to be amorphous or too small to result in

diffraction peaks. Similar to the blend with A4, the A5 blend film roughens upon annealing

with large waves spanning microns across the substrate. The roughening is likely formation

of the crystalline domains that give rise to the broadened diffraction peak observed in the

XRD pattern in Figure 5.13 k).

87

Figure 5.14. 2D AFM topographical images of the surfaces of 1:1 thin-film blends of D1

with acceptors A1, A2, A3, A4 and A5, as-cast and after annealing at 180 °C for 10 min.

88

5.4.4. Organic Solar Cell Testing, Part II

To compare the PV performance of each new donor-acceptor system as the active

layer in OSCs, a batch of devices was fabricated on five different substrates with a

conventional device architecture. The fabrication procedure is outlined in Section 5.2.5.

The metal cathode was deposited onto the unannealed (i.e., as-cast) active layers. All

devices were tested as-cast and then annealed on a hotplate at 180 °C for 10 min in the

glovebox before re-testing the annealed devices. The J-V curves of all devices were linear

with very high series resistance and minimal current; no effect was observed from

annealing. However, only one batch of devices was made for these D-A combinations.

Since the control devices with (p-DTS(FBTTh2)2 and PCBM as the donor and acceptor,

respectively) had poorer PCEs than in previous batches, it is possible that other materials

in the cells (e.g., PEDOT:PSS) could have been defective.

5.4.5. Conclusions, Part II

In Part II of this project, it was demonstrated that molecular design (e.g., R-groups

and naphthalimide units) influences the electronic and film-forming properties of a series

of DPP-based small-molecule derivatives paired with a 3D TPA-based donor.

Based on the absorption spectra, XRD and AFM images, conclusions can be drawn

about the effect of R-groups on the self-assembly the materials within solid-state thin-film

blends. Among the five acceptor derivatives studied, A1, A2 and A3 clearly respond to

heat most significantly as indicated by i) changes in the absorption properties (UV-Vis),

and ii) development of large crystalline domains (XRD and AFM) upon annealing. These

conditions are not favourable for organic photovoltaic applications – the morphology needs

to be robust and resistant to heat in functional solar cells since the cells will increase in

temperature during regular cell operation under full-sun exposure.

89

In contrast, the solid-state ordering of A4 appears to be quite resistant to heat. It

can be speculated that the substitution of the napthalimide unit for phthalimide reduces the

ordering of the molecules and is responsible for the lack of crystallinity in the film.

However, the lack of order in the thin-film may not be ideal; if no ordered domains exist,

charge-transport pathways will not be formed and no current will be generated by the cell.

In this case, charge-mobility measurements would be needed for confirmation.

A5 exhibits the “best-of-both-worlds” scenario: the film is amorphous as-cast, but

develops crystalline networks once annealed at high-temperature; a processing condition

that can easily be implemented into the fabrication of cells on an industrial scale. These

properties suggest that the blend contains charge-transporting percolation networks and that

the morphology and electronics will remain in-tact under normal cell operation.

5.5. Outlook and Future Work

Ultimately, fabrication employing each of the blends using a conventional-style

architecture yielded non-functional organic solar cell devices. The reason for the poor

performance and low current is unknown based on the current data. Nevertheless, several

approaches exist for troubleshooting non-functioning systems:

i) Device engineering – investigating alternative device architectures. For

example, other members of the Hill lab have discovered that “inverted” solar

cell designs can perform better than conventional cells when using the same

active layer blends.

ii) Applying additional experimental techniques to further characterize the blends

and provide a better understanding of the materials. For example, charge-

transport properties of the materials (e.g., hole-mobility) of D1 and D2 by

fabricating thin-film transistors (TFTs) are important to understanding the cause

90

of the low photocurrents generated by the preliminary devices. It is also of

interest to investigate the thermal stability of the materials using TGA,

especially when high-temperature annealing is used as a processing technique.

These data are also relevant to the application of D2/HTM02 as the hole-

transporting layer in PSCs (Chapter 4).

iii) Synthesizing more acceptor derivatives with different R-groups. The organic

semiconductors presented in this study are infinitely tunable – using the results

gathered in this study and the trends observed in the effect of side-chain

engineering on the electronic and structural properties, efforts can now be

focused on derivatives with the best properties. For example, it has been shown

that branched chains at the DPP core effectively reduce crystallinity and domain

sizes in solid-state thin films.

iv) Continuing to optimize processing conditions of the existing materials to tune

the blend properties. This strategy includes the application of techniques such

as solvent additives, solvent vapour annealing and inert polymer additives.

Preliminary investigations of solvent additives and solvent vapour annealing

were performed and are discussed in Appendix J.

v) Substitution of other functionalities. For example, A1 has three types of

functional units: i) DPP, ii) thiophene and iii) phthalimide. Each of these units

can be replaced by analogous units (e.g., furan) that can alter both the electronic

properties (e.g., absorption, bandgap, etc.) and self-assembly (e.g., d-spacing,

crystallinity, domain sizes, etc.).

While each of these five possible directions holds promise, strategies iv) and v) were

selected while continuing to work with similar materials. It is clear that the DPP-based

91

compounds demonstrate interesting electronic and structural responses to processing

conditions. For this reason, a related compound (i.e., AX in Figure 5.15) was selected as

an electron donor paired with a promising non-fullerene acceptor. Strategies outlined in

iv) were applied to this new system. Solution-processing techniques were shown to change

the electronic, structural and morphological properties of thin-films and could be used to

enhance device performance when applied to the fabrication of organic solar cells.

Figure 5.15. A comparison of the structures of A2 and AX with thiophene and furan units

highlighted in red and green, respectively.

92

CHAPTER 6 – CONCLUSIONS

The first step in the program was to design a fabrication procedure for high-

efficiency perovskite solar cells (Chapter 3). Reproducible formation of dense, uniform

perovskite films via the dip-coating method is inherently challenging. Methods that

increased control over the perovskite conversion step (e.g., deposition via sequential spin-

coating) ultimately improved both film quality and reproducibility, leading to better control

devices. The first goal in the project (see Section 1.6) was achieved; the best-performing

devices fabricated using the established procedure achieved a power conversion efficiency

of 13.43% (see Appendix F), which is comparable to reports made in the literature.59

Development of a detailed protocol for perovskite solar cell fabrication was important for

not only these projects, but for future endeavours in the Welch and Hill laboratories

including the development of novel organic hole-transporting materials.

Once a reproducible fabrication procedure for efficient perovskite solar cells was

established, new economical hole-transporting materials were investigated to address the

issue of the high-cost of spiro-OMeTAD (Chapter 4). Triphenylamine-based hole-

transporting compounds (HTM01, HTM02 and HTM03) were selected along with a well-

known small molecule donor (HTM04) used in high-performance organic solar cells. The

simplest hole-transporting material (HTM01) was shown to give the best performance in

perovskite solar cells among the triphenylamine compounds. The bandgap of the hole-

transporting materials was shown to have little effect on device performance. For example,

HTM03 and HTM04 both have narrow bandgaps yet display significantly different

performance. Compared to the triphenylamine-based HTMs, spiro-OMeTAD formed the

smoothest film over the perovskite active layer, suggesting that it provided better coverage

and may explain its better performance as hole-transporting material in perovskite solar

93

cells. In order to achieve the goal of developing a low-cost and efficient HTM for

perovskite solar cells using the triphenylamine core, efforts should be focused on increasing

solubility in order cast thicker and more uniform hole-transporting layers.

Despite the relatively poor performance of the custom-built triphenylamine-based

compounds as HTMs in perovskite solar cells, they demonstrate potential as donors in

organic solar cells (Chapter 5). The TPA hole-transporting compounds were paired with

custom-built acceptors to create fullerene-free donor-acceptor pairs. By characterizing the

absorption properties (using UV-Vis spectroscopy) and energy levels (using cyclic

voltammetry), organic materials could be assessed for compatibility as donors and

acceptors in organic solar cells. The final goal of the project was achieved; two non-

fullerene materials were selected (i.e., D1 and A1) that met the three of the five criteria for

a promising donor-acceptor pair. Thermal annealing was shown to have an effect on both

the electronic and structural properties of D1-A1 blend films. However, the morphologies

of the D1-A1 thin films were not appropriate for organic solar cell active layers; the large

domain sizes contributed to the poor PV performance. To address the challenge of

achieving thin films with a good morphology, new 2D acceptors were tested for

compatibility with D1. Molecular design and alkyl chain modifications were shown to

significantly change the self-assembly of the A1 derivatives in solid-state thin-film

systems. Although these thin film blends were shown to give poor PV performance, the

design strategy demonstrates the tunability of organic semiconductors and ease of

modifying the properties of organic thin-films.

In summary, the processing conditions used for thin-film formation affects

electronic, structural and morphological properties which ultimately influence device

performance in solution-processed perovskite and organic solar cells. Understanding how

94

spin-coating techniques influence the thin-film properties of both perovskites and organic

semiconductors is necessary for i) optimization of solar cells and ii) development of new

functional materials. Through further investigation of processing conditions and

development of new derivatives of organic semiconductors, low-cost and efficient solution-

processed perovskite and organic solar cells can be realized.

95

REFERENCES

1 US Energy Information Administration, Annual Energy Outlook 2015,

Washington, DC, 2015.

2 N. S. Lewis and D. G. Nocera, Proc. Natl. Acad. Sci., 2006, 103, 15729–15735.

3 US Department of Energy, 2013.

4 J. Yan and B. R. Saunders, RSC Adv., 2014, 4, 43286–43314.

5 F. C. Krebs, T. Tromholt and M. Jørgensen, Nanoscale, 2010, 2, 873–86.

6 A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka, J. Am. Chem. Soc., 2009, 131,

6050–6051.

7 H. Zhou, Q. Chen, G. Li, S. Luo, T. Song, H.-S. Duan, Z. Hong, J. You, Y. Liu and

Y. Yang, Science, 2014, 345, 542–546.

8 M. A. Green, K. Emery, Y. Hishikawa, W. Warta and E. D. Dunlop, Prog.

Photovoltaics Res. Appl., 2015, 23, 1–9.

9 C. W. Tang, Appl. Phys. Lett., 1986, 48, 183.

10 G. C. Papavassiliou, G. A. Mousdis and I. B. Koutselas, Adv. Mater. Opt.

Electron., 1999, 9, 265–271.

11 T. Ishihara, J. Lumin., 1994, 60, 269–274.

12 Y. Kawamura, H. Mashiyama and K. Hasebe, J. Phys. Soc. Japan, 2002, 71, 1694–

1697.

13 T. Ishihara, J. Takahashi and T. Goto, Phys. Rev. B, 1990, 42, 11099–11107.

14 J. Calabrese, N. L. Jones, R. L. Harlow, N. Herron, D. L. Thorn and Y. Wang, J.

Am. Chem. Soc., 1991, 113, 2328–2330.

15 D. G. Billing and A. Lemmerer, CrystEngComm, 2007, 9, 236–244.

16 K. Tanaka, T. Takahashi, T. Ban, T. Kondo, K. Uchida and N. Miura, Solid State

Commun., 2003, 127, 619–623.

17 H.-S. Kim, C.-R. Lee, J.-H. Im, K.-B. Lee, T. Moehl, A. Marchioro, S.-J. Moon, R.

Humphry-Baker, J.-H. Yum, J. E. Moser, M. Gratzel and N.-G. Park, Sci. Rep.,

2012, 2, 1–7.

96

18 C. C. Stoumpos, C. D. Malliakas and M. G. Kanatzidis, Inorg. Chem., 2013, 52,

9019–9038.

19 M. Samiee, S. Konduri, B. Ganapathy, R. Kottokkaran, H. A. Abbas, A. Kitahara,

P. Joshi, L. Zhang, M. Noack and V. Dalal, Appl. Phys. Lett., 2014, 105.

20 C. S. Ponseca, T. J. Savenije, M. Abdellah, K. Zheng, A. Yartsev, T. Pascher, T.

Harlang, P. Chabera, T. Pullerits, A. Stepanov, J. P. Wolf and V. Sundström, J.

Am. Chem. Soc., 2014, 136, 5189–5192.

21 S. Avasthi, W. E. McClain, G. Man, A. Kahn, J. Schwartz and J. C. Sturm, Appl.

Phys. Lett., 2013, 102, 203901.

22 A. Marchioro, J. Teuscher, D. Friedrich, M. Kunst, R. van de Krol, T. Moehl, M.

Gratzel and J.-E. Moser, Nat. Phot., 2014, 8, 250–255.

23 J.-Y. Jeng, Y.-F. Chiang, M.-H. Lee, S.-R. Peng, T.-F. Guo, P. Chen and T.-C.

Wen, Adv. Mater., 2013, 25, 3727–3732.

24 O. Malinkiewicz, A. Yella, Y. H. Lee, G. M. Espallargas, M. Graetzel, M. K.

Nazeeruddin and H. J. Bolink, Nat. Phot., 2014, 8, 128–132.

25 Q. Xue, Z. Hu, J. Liu, J. Lin, C. Sun, Z. Chen, C. Duan, J. Wang, C. Liao, W. M.

Lau, F. Huang, H.-L. Yip and Y. Cao, J. Mater. Chem. A, 2014, 2, 19598–19603.

26 J. You, Z. Hong, Y. Yang, Q. Chen, M. Cai, T.-B. Song, C.-C. Chen, S. Lu, Y. Liu

and H. Zhou, ACS Nano, 2014, 8, 1674–1680.

27 Y. Sun, G. C. Welch, W. L. Leong, C. J. Takacs, G. C. Bazan and A. J. Heeger,

Nat. Mater., 2012, 11, 44–48.

28 L. J. A. Koster, S. E. Shaheen and J. C. Hummelen, Adv. Energy Mater., 2012, 2,

1246–1253.

29 B. A. Gregg and M. C. Hanna, J. Appl. Phys., 2003, 93, 3605.

30 I. G. Hill, A. Kahn, Z. G. Soos and R. A. Pascal, Jr, Chem. Phys. Lett., 2000, 327,

181–188.

31 A. K. K. Kyaw, D. H. Wang, V. Gupta, J. Zhang, S. Chand, G. C. Bazan and A. J.

Heeger, Adv. Mater., 2013, 25, 2397–2402.

32 G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science, 1995, 270,

1789–1791.

97

33 A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson, Chem. Rev., 2010,

110, 6595–6663.

34 a. J. Oostra, E. Smits, D. de Leeuw, P. Blom and J. Michels, Phys. Chem. Chem.

Phys., 2015, 17, 21501–21506.

35 F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2009, 93, 394–412.

36 C. W. Extrand, Polym. Eng. Sci., 1994, 34, 390.

37 S. J. L. Billinge and I. Levin, Science, 2007, 316, 561–5.

38 N. Maskil and M. Deutsch, Phys. Rev. A, 1988, 37, 2947–2952.

39 A. Shafiee, M. M. Salleh and M. Yahaya, Sains Malaysiana, 2011, 40, 173–176.

40 M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami and H. J. Snaith, Science,

2012, 338, 643–647.

41 J. M. Ball, M. M. Lee, A. Hey and H. J. Snaith, Energy Environ. Sci., 2013, 6,

1739–1743.

42 P. Docampo, J. M. Ball, M. Darwich, G. E. Eperon and H. J. Snaith, Nat.

Commun., 2013, 4, 1–6.

43 G. E. Eperon, V. M. Burlakov, P. Docampo, A. Goriely and H. J. Snaith, Adv.

Funct. Mater., 2014, 24, 151–157.

44 S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. P. Alcocer, T.

Leijtens, L. M. Herz, A. Petrozza and H. J. Snaith, Science, 2013, 342, 341–344.

45 G. C. Xing, N. Mathews, S. Y. Sun, S. S. Lim, Y. M. Lam, M. Gratzel, S.

Mhaisalkar and T. C. Sum, Science, 2013, 342, 344–347.

46 D. Liu and T. L. Kelly, Nat. Phot., 2014, 8, 133–138.

47 M. Liu, M. B. Johnston and H. J. Snaith, Nat., 2013, 501, 395–398.

48 K. Hwang, Y. Jung, Y. Heo, F. H. Scholes, S. E. Watkins, J. Subbiah, D. J. Jones,

D. Kim and D. Vak, Adv. Mater., 2015, 27, 1241–1247.

49 M. Xiao, F. Huang, W. Huang, Y. Dkhissi, Y. Zhu, J. Etheridge, A. Gray-Weale,

U. Bach, Y.-B. Cheng and L. Spiccia, Angew. Chemie, 2014, 126, 10056–10061.

50 K. Liang, D. B. Mitzi and M. T. Prikas, Chem. Mater., 1998, 10, 403–411.

98

51 J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin

and M. Gratzel, Nat., 2013, 499, 316–319.

52 N. J. Jeon, J. H. Noh, Y. C. Kim, W. S. Yang, S. Ryu and S. Il Seok, Nat. Mater.,

2014, 13, 897–903.

53 Z. Xiao, C. Bi, Y. Shao, Q. Dong, Q. Wang, Y. Yuan, C. Wang, Y. Gao and J.

Huang, Energy Environ. Sci., 2014, 7, 2619–2623.

54 B.-E. Cohen, S. Gamliel and L. Etgar, Apl Mater., 2014, 2, 081502.

55 P. A. Beckmann, Cryst. Res. Technol., 2010, 45, n/a–n/a.

56 J. Yang, B. D. Siempelkamp, D. Liu and T. L. Kelly, ACS Nano, 2015, 9, 1955–63.

57 A. Dualeh, N. Tétreault, T. Moehl, P. Gao, M. K. Nazeeruddin and M. Grätzel,

Adv. Funct. Mater., 2014, 24, 3250–3258.

58 J.-H. Im, I.-H. Jang, N. Pellet, M. Grätzel and N.-G. Park, Nat. Nano., 2014, 9,

927–932.

59 C. Bi, Y. Shao, Y. Yuan, Z. Xiao, C. Wang, Y. Gao and J. Huang, J. Mater. Chem.

A, 2014, 2, 18508–18514.

60 O. Knop, R. E. Wasylishen, M. A. White, T. S. Cameron and M. J. M. Vanoort,

Can. J. Chem. , 1990, 68, 412–422.

61 A. Dualeh, P. Gao, S. Il Seok, M. K. Nazeeruddin and M. Grätzel, Chem. Mater.,

2014, 26, 6160–6164.

62 T. Baikie, Y. Fang, J. M. Kadro, M. Schreyer, F. Wei, S. G. Mhaisalkar, M.

Graetzel and T. J. White, J. Mater. Chem. A, 2013, 1, 5628–5641.

63 N. J. Jeon, H. G. Lee, Y. C. Kim, J. Seo, J. H. Noh, J. Lee and S. Il Seok, J. Am.

Chem. Soc., 2014, 136, 7837–7840.

64 C. He, Q. He, Y. Yi, G. Wu, F. Bai, Z. Shuai and Y. Li, J. Mater. Chem., 2008, 18,

4085.

65 S. Allard, M. Forster, B. Souharce, H. Thiem and U. Scherf, Angew. Chemie - Int.

Ed., 2008, 47, 4070–4098.

66 P. Strohriegl and J. V. Grazulevicius, Adv. Mater., 2002, 14, 1439–1452.

67 Z. Li, T. Ye, S. Tang, C. Wang, D. Ma and Z. Li, J. Mater. Chem. C, 2015, 3,

2016–2023.

99

68 J. J. Wang, S. R. Wang, X. G. Li, L. F. Zhu, Q. B. Meng, Y. Xiao and D. M. Li,

Chem. Commun., 2014, 50, 5829–5832.

69 J. H. Heo, S. H. Im, J. H. Noh, T. N. Mandal, C. S. Lim, J. A. Chang, Y. H. Lee, H.

J. Kim, A. Sarkar, M. K. Nazeeruddin, M. Gratzel and S. I. Seok, Nat. Photonics,

2013, 7, 487–492.

70 H. Choi, S. Park, S. Paek, P. Ekanayake, M. K. Nazeeruddin and J. Ko, J. Mater.

Chem. A, 2014, 2, 19136–19140.

71 S. Do Sung, M. S. Kang, I. T. Choi, H. M. Kim, H. Kim, M. Hong, H. K. Kim and

W. I. Lee, Chem. Commun., 2014, 50, 14161–14163.

72 T. Malinauskas, M. Daskeviciene, G. Bubniene, I. Petrikyte, S. Raisys, K.

Kazlauskas, V. Gaidelis, V. Jankauskas, R. Maldzius, S. Jursenas and V. Getautis,

Chem. Eur. J., 2013, 19, 15044–15056.

73 N. Satoh, T. Nakashima and K. Yamamoto, J. Am. Chem. Soc., 2005, 127, 13030–

13038.

74 D. J. Burke and D. J. Lipomi, Energy Environ. Sci., 2013, 6, 2053–2066.

75 M. N. Belgacem and A. Gandini, Monomers, polymers and composites from

renewable resources, Elsevier, 2011.

76 O. Gidron, A. Dadvand, Y. Sheynin, M. Bendikov and D. F. Perepichka, Chem.

Commun., 2011, 47, 1976–1978.

77 A. D. Hendsbee, J.-P. Sun, T. M. McCormick, I. G. Hill and G. C. Welch, Org.

Electron., 2015, 18, 118–125.

78 K. Oniwa, T. Kanagasekaran, T. Jin, M. Akhtaruzzaman, Y. Yamamoto, H.

Tamura, I. Hamada, H. Shimotani, N. Asao, S. Ikeda and K. Tanigaki, J. Mater.

Chem. C, 2013, 1, 4163–4170.

79 J. Liu, B. Walker, A. Tamayo, Y. Zhang and T.-Q. Nguyen, Adv. Funct. Mater.,

2013, 23, 47–56.

80 A. B. Tamayo, M. Tantiwiwat, B. Walker and T.-Q. Nguyen, J. Phys. Chem. C,

2008, 112, 15543–15552.

81 J. A. Love, C. M. Proctor, J. Liu, C. J. Takacs, A. Sharenko, T. S. van der Poll, A.

J. Heeger, G. C. Bazan and T.-Q. Nguyen, Adv. Funct. Mater., 2013, 23, 5019–

5026.

100

82 T. S. van der Poll, J. A. Love, T.-Q. Nguyen and G. C. Bazan, Adv. Mater., 2012,

24, 3646–3649.

83 Y. Zhao, A. M. Nardes and K. Zhu, Appl. Phys. Lett., 2014, 104, 21306.

84 J. Meyer, S. Hamwi, M. Kröger, W. Kowalsky, T. Riedl and A. Kahn, Adv. Mater.,

2012, 24, 5408–27.

85 A. Abrusci, S. D. Stranks, P. Docampo, H.-L. Yip, A. K. Y. Jen and H. J. Snaith,

Nano Lett., 2013, 13, 3124–3128.

86 M. G. Helander, M. T. Greiner, Z. B. Wang, W. M. Tang and Z. H. Lu, J. Vac. Sci.

& Technol. A, 2011, 29, 011019.

87 G. C. Welch, L. A. Perez, C. V. Hoven, Y. Zhang, X.-D. Dang, A. Sharenko, M. F.

Toney, E. J. Kramer, T.-Q. Nguyen and G. C. Bazan, J. Mater. Chem., 2011, 21,

12700.

88 M. Cheng, C. Chen, X. Yang, J. Huang, F. Zhang, B. Xu and L. Sun, Chem.

Mater., 2015, 27, 1808–1814.

89 L. A. Perez, J. T. Rogers, M. A. Brady, Y. Sun, G. C. Welch, K. Schmidt, M. F.

Toney, H. Jinnai, A. J. Heeger, M. L. Chabinyc, G. C. Bazan and E. J. Kramer,

Chem. Mater., 2014, 26, 6531–6541.

90 R. B. Ross, C. M. Cardona, D. M. Guldi, S. G. Sankaranarayanan, M. O. Reese, N.

Kopidakis, J. Peet, B. Walker, G. C. Bazan, E. Van Keuren, B. C. Holloway and

M. Drees, Nat. Mater., 2009, 8, 208–212.

91 Y. He, H.-Y. Chen, J. Hou and Y. Li, J. Am. Chem. Soc., 2010, 132, 1377–1382.

92 J. Roncali, P. Leriche and A. Cravino, Adv. Mater., 2007, 19, 2045–2060.

93 D. M. Guldi, Chem. Commun., 2000, 321–327.

94 L. A. Perez, K. W. Chou, J. A. Love, T. S. van der Poll, D.-M. Smilgies, T.-Q.

Nguyen, E. J. Kramer, A. Amassian and G. C. Bazan, Adv. Mater., 2013, 25,

6380–6384.

95 J.-Y. Pan, L.-J. Zuo, X.-L. Hu, W.-F. Fu, M.-R. Chen, L. Fu, X. Gu, H.-Q. Shi, M.-

M. Shi, H.-Y. Li and H.-Z. Chen, ACS Appl. Mater. Interfaces, 2013, 5, 972–980.

96 J. Zhang, D. Deng, C. He, Y. He, M. Zhang, Z.-G. Zhang, Z. Zhang and Y. Li,

Chem. Mater., 2011, 23, 817–822.

101

97 J. Zhang, Y. Yang, C. He, D. Deng, Z. Li and Y. Li, J. Phys. D. Appl. Phys., 2011,

44, 475101.

98 H. Shang, H. Fan, Y. Liu, W. Hu, Y. Li and X. Zhan, Adv. Mater., 2011, 23, 1554–

1557.

99 A. D. Hendsbee, J.-P. Sun, L. R. Rutledge, I. G. Hill and G. C. Welch, J. Mater.

Chem. A, 2014, 2, 4198–4207.

100 Y. Kim, A. Ballantyne, J. Nelson and D. Bradley, Org. Electron., 2009, 10, 205–

209.

101 S. Qu and H. Tian, Chem. Commun. (Camb)., 2012, 48, 3039–51.

102 Z. Du, W. Chen, Y. Chen, S. Qiao, X. Bao, S. Wen, M. Sun, L. Han and R. Yang,

J. Mater. Chem. A, 2014, 2, 15904–15911.

103 B. Kan, Q. Zhang, M. Li, X. Wan, W. Ni, G. Long, Y. Wang, X. Yang, H. Feng

and Y. Chen, J. Am. Chem. Soc., 2014, 136, 15529–15532.

104 Z. Yi, W. Ni, Q. Zhang, M. Li, B. Kan, X. Wan and Y. Chen, J. Mater. Chem. C,

2014, 2, 7247–7255.

105 J. W. Ryan and Y. Matsuo, Sci. Rep., 2015, 5.

106 J.-H. Im, C.-R. Lee, J.-W. Lee, S.-W. Park and N.-G. Park, Nanoscale, 2011, 3,

4088–4093.

107 H. Li, K. Fu, A. Hagfeldt, M. Grätzel, S. G. Mhaisalkar and A. C. Grimsdale,

Angew. Chemie - Int. Ed., 2014, 53, 4085–4088.

108 S. Sun, T. Salim, N. Mathews, M. Duchamp, C. Boothroyd, G. Xing, T. C. Sum

and Y. M. Lam, Energy Environ. Sci., 2014, 7, 399–407.

109 H. Li, K. Fu, P. P. Boix, L. H. Wong, A. Hagfeldt, M. Grätzel, S. G. Mhaisalkar

and A. C. Grimsdale, ChemSusChem, 2014, 7, 3420–3425.

110 L. Zheng, Y. S. Chung, Y. Ma, L. Zhang, L. Xiao, Z. Chen, S. Wang, B. Qu and Q.

Gong, Chem. Commun., 2014, 1, 1–4.

111 D. Bi, L. Yang, G. Boschloo, A. Hagfeldt and E. M. J. Johansson, J. Phys. Chem.

Lett., 2013, 4, 1532–1536.

112 F. Matteocci, S. Razza, F. Di Giacomo, S. Casaluci, G. Mincuzzi, T. M. Brown, A.

D’Epifanio, S. Licoccia and A. Di Carlo, Phys. Chem. Chem. Phys., 2014, 16,

3918–3923.

102

113 Y. S. Kwon, J. Lim, H.-J. Yun, Y.-H. Kim and T. Park, Energy Environ. Sci.,

2014, 7, 1454–1460.

114 H. J. Snaith and M. Gratzel, Appl. Phys. Lett., 2006, 89, 262113–262114.

115 A. Abate, T. Leijtens, S. Pathak, J. Teuscher, R. Avolio, M. E. Errico, J.

Kirkpatrik, J. M. Ball, P. Docampo, I. McPherson and H. J. Snaith, Phys. Chem.

Chem. Phys., 2013, 15, 2572–2579.

116 W. H. Howie, J. E. Harris, J. R. Jennings and L. M. Peter, Sol. Energy Mater. Sol.

Cells, 2007, 91, 424–426.

117 Y. Huang, W. Wen, S. Mukherjee, H. Ade, E. J. Kramer and G. C. Bazan, Adv.

Mater., 2014, 26, 4168–4172.

118 H. Li, T. Earmme, G. Ren, A. Saeki, S. Yoshikawa, N. M. Murari, S.

Subramaniyan, M. J. Crane, S. Seki and S. A. Jenekhe, J. Am. Chem. Soc., 2014,

136, 14589–14597.

119 W. Jiang, L. Ye, X. Li, C. Xiao, F. Tan, W. Zhao, J. Hou and Z. Wang, Chem.

Commun., 2014, 50, 1024–1026.

120 Y. Zang, C.-Z. Li, C.-C. Chueh, S. T. Williams, W. Jiang, Z.-H. Wang, J.-S. Yu

and A. K.-Y. Jen, Adv. Mater., 2014, 26, 5708–14.

121 J. Zhao, Y. Li, H. Lin, Y. Liu, K. Jiang, C. Mu, T. Ma, J. Y. Lin Lai, H. Hu, D. Yu

and H. Yan, Energy Environ. Sci., 2015, 8, 520–525.

122 W. Chen, X. Yang, G. Long, X. Wan, Y. Chen and Q. Zhang, J. Mater. Chem. C,

2015, 3, 4698–4705.

123 S.-Y. Liu, C.-H. Wu, C.-Z. Li, S.-Q. Liu, K.-H. Wei, H.-Z. Chen and A. K.-Y. Jen,

Adv. Sci., 2015, 2, 1500014.

124 Y. Liu, C. Mu, K. Jiang, J. Zhao, Y. Li, L. Zhang, Z. Li, J. Y. L. Lai, H. Hu, T. Ma,

R. Hu, D. Yu, X. Huang, B. Z. Tang and H. Yan, Adv. Mater., 2015, 27, 1014–

1014.

125 Y. Lin, P. Cheng, Y. Li and X. Zhan, Chem. Commun., 2012, 48, 4773–4775.

126 Y. Lin, Y. Wang, J. Wang, J. Hou, Y. Li, D. Zhu and X. Zhan, Adv. Mater., 2014,

26, 5137–5142.

127 A. Sharenko, D. Gehrig, F. Laquai and T.-Q. Nguyen, Chem. Mater., 2014, 26,

4109–4118.

103

128 A. Sharenko, C. M. Proctor, T. S. van der Poll, Z. B. Henson, T.-Q. Nguyen and G.

C. Bazan, Adv. Mater., 2013, 25, 4403–6.

129 R. Singh, E. Aluicio-Sarduy, Z. Kan, T. Ye, R. C. I. MacKenzie and P. E.

Keivanidis, J. Mater. Chem. A, 2014, 2, 14348–14353.

130 M. Li, J. Liu, X. Cao, K. Zhou, Q. Zhao, X. Yu, R. Xing and Y. Han, Phys. Chem.

Chem. Phys., 2014, 16, 26917–26928.

104

APPENDIX A – SOLUTION PREPARATIONS

TiO2 Precursor Solution Preparation

To form the compact TiO2 layer, a precursor solution containing titanium (IV)

isopropoxide (Ti(iPrO)4), purchased from Sigma-Aldrich, Product No. 377996-25ML,

99.999% purity) was prepared using a modified procedure outlined by Abrusci et al.85 Two

solutions were prepared in separate 8 mL vials. The first vial contained 2530 μL of ethanol

and 35 μL of a stock solution of 2.0 M HCl (prepared in air). The second solution contained

2530 μL of ethanol and 370 μL of Ti(iPrO)4 (prepared in the glovebox). The first solution

was added dropwise to the second solution while the second was stirred in air to form the

final sol-gel solution.

PbI2 Solution Preparation

A 1.0 M PbI2 solution was prepared by weighing 461 mg of PbI2 (purchased from

Alfa-Aesar, Product No. 12724, 99.9985% purity) in a 4- or 8-mL Teflon-capped vial and

adding 1000 μL of dry DMF and a stir bar. The solid was dissolved by heating and stirring

on a hotplate and maintained at the desired temperature throughout the deposition

procedure. The temperature of the solution was recorded by attaching a thermocouple

probe to the outer wall of the vial. The 0.5 M and 0.75 M PbI2 solutions were prepared in

a similar fashion, but dissolved by stirring at RT.

Spiro-OMeTAD (Doped) Solution Preparation

The HTM solution was prepared using the recipe described by Liu et al.46: 80 mg

of spiro-OMeTAD (purchased from Sigma-Aldrich, Product No. 792071-1G, 99% purity),

28.5 µL of 4-tert-butylpyridine (tBP, purchased from Sigma-Aldrich, Product No. 142379-

25G, 96% purity) and 17.5 µL of a 520 mg/mL solution of lithium-

bis(trifluoromethane)sulfonimide (Li-TFSI, purchased from Sigma-Aldrich, Product No.

105

544094-5G, 99.95% purity) in acetonitrile was combined and dissolved 1 mL of

chlorobenzene. The solid spiro-OMeTAD was weighed onto a weigh paper and then

transferred to a 4.0 mL Teflon®-capped vial. tBP, chlorobenzene and the Li-TFSI solution

were transferred separately into the vial using micropipettes. The mixture was dissolved

using a stir-bar and stirring at RT.

106

APPENDIX B – SYNTHESIS OF CH3NH3I

The synthesis of CH3NH3I was performed using the procedure documented by Im

et al. as a guideline.106 A 250 mL round-bottom flask was loaded with 27 mL of 55 wt-%

HI(aq) (BC Scientific, Product No. 0152-01), 30 mL of a 40% aqueous solution of

methylamine (Fisher Scientific, Product No. 00983) and a stir bar. The reaction mixture

was stirred in an ice bath in air for 3 h. The solvent was removed using a rotary evaporator,

leaving behind a brown solid. The solid product was slurried in diethyl ether for 30 minutes

and filtered using a Buchner funnel. This process was repeated twice more to wash the

product. The washed final product was recrystallized from a minimum amount of ethanol,

cooled, and filtered to recover white crystals. The crystals were washed with diethyl ether

and dried in a vacuum oven overnight. The solid crystalline product was stored in an Ar-

filled glovebox prior to use.

107

APPENDIX C – OTHER FACTORS INFLUENCING LEAD IODIDE FILM

MORPHOLOGY

Solution Temperature (Tsolution)

It was determined that the minimum value of Tsolution for complete dissolution of a

1.0 M solution of PbI2 in DMF was 50 °C. Increasing Tsolution above the threshold

temperature (i.e., 50 °C) results in perovskite films with smaller crystallites (Figure C1).

However, there are no significant differences between the films cast from solutions held at

70, 90, 110, and 130 °C in terms of crystallite size and distribution.

Figure C1. The effect of solution temperature. SEM images of PbI2 films spin-cast on

TiO2 compact films at different temperatures and the corresponding CH3NH3PbI3 films

after conversion via dipping. The solution was spin-coated using a spin speed of 3000 rpm

for 30 s onto TiO2-coated substrates heated at 100 °C.

The Effect of Atmosphere

By depositing onto a hot substrate or performing the deposition under an inert (i.e.,

Ar) atmosphere, film coverage can be drastically improved. In both cases, the films exhibit

smaller grain sizes and better coverage than the unheated film in ambient. It is also

observed that the film spun under ambient conditions with a heated substrate had a lower

frequency of pinholes than the one spun in the glovebox (see Figure C2). This

demonstrates that inert conditions are not required for uniform film formation – simply

108

heating the substrate prior to deposition is an effective method for hindering crystal growth

and increasing film coverage.

Figure C2. The effect of atmosphere on PbI2 film formation. Depicted are SEM images

of PbI2 and corresponding CH3NH3PbI3 films spin-cast from a 1.0 M PbI2 solution in DMF

held at 70 °C. The solution was spin-coated in either air (ambient) or N2 (inert) using a

spin speed of 1000 rpm for 30 s onto substrates held at either R.T. or 100 °C.

109

APPENDIX D – PEROVSKITE X-RAY DIFFRACTION PEAK

INTENSITIES

Table D1. Peak areas corresponding to XRD plots in Figure 3.15, determined by

applying the Pearson VII function and using Jade software.

[MAI] Peak

Area for

PbI2

(001)

PbI2

(001)

Error

Peak

Area for

MAPbI3

(110)

MAPbI3

(110)

Error

IPbI2/(IPbI2 +

IMAPbI3)

IPbI2/(IPbI2 +

IMAPbI3)

Error

20 5662.7 37.8 2319.9 26.7 0.70938 0.010559

30 3760.1 32.3 2636.1 26.4 0.587865 0.009255

40 1933.3 25.6 3833.2 31.1 0.335264 0.006842

50 71.4 12.5 4592.7 N/A 0.015308 0.00379

For simplicity, let:

IPbI2/(IPbI2 + IMAPbI3) = A

Error for IPbI2/(IPbI2 + IMAPbI3) = δA

IPbI2 = B

Error for IPbI2 = δB

IMAPbI3 = C

Error for C = δC

Calculation of error:

δA = |A| (2*( δB/B)2 + (δC/C)2)1/2

110

APPENDIX E – THIOPHENE IN ORGANIC SMALL MOLECULES

Thiophene heterocyclic derivatives are important electron-donating (D)

functionalities in organic HTMs. There are a number of reports of high-performance small-

molecule HTMs designed for applications in PSCs based on thiophene derivatives – the

molecular structure of three examples are given in Figure E1. In the first example, a simple

HTM known as “H101” was synthesized and shown to achieve a respectable PCE of 10.6

% without the use of dopants. In this molecule, 3,4-ethylenedioxythiophene (EDOT)

core107 was used in an shown to produce higher currents than unsubstituted benzene or

thiophene. EDOT is a common functional group in HTMs, with a popular example being

the mixture of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS).

Although mostly used for OPVs, PEDOT:PSS is now gaining attention as the HTL in

inverted PSCs.23,24,108 In a subsequent paper, the same group reported a derivative of H101

with a tetra-substituted thiophene core (aka “H111”) that achieved a PCE of 14.9 %;109 to

date, it is the highest reported PCE for PSCs without Spiro-OMeTAD. A third example is

a thiophene oligomer with benzodithiophene (BDT) core, named “DR3TBDTT”.110 From

DFT studies the HOMO is located at BDT unit, and the LUMO occupies the electron-

drawing ethylrhodanine groups at both ends, suggesting that that hole-transport may be

occurring primarily at the core in this acceptor–donor–acceptor backbone electronic

structure. Three thiophene bridging units extend conjugation from the core to the ends.

Unlike the previous two examples, devices with DR3TBDTT performed worse when doped

with Li-TFSI and tBP, suggesting that structural differences are a factor in the response to

such dopants.

111

Figure E1. Structures of small-molecule HTMs for PSCs with thiophene functionalities

highlighted in red.

Polymer HTMs with thiophene units also have precedence for applications in

PCEs. Three examples are shown in Figure E2. P3HT, a narrow band-gap

semiconductor with a successful history as a donor in OSCs has been shown to yield

efficiencies as the HTL in PSCs, with PCEs of up to 6.7%.69,111,112 Although P3HT has

excellent hole mobility, the major disadvantage of using it as an HTM in PSCs is its

elevated HOMO level – corresponding devices have lower Vocs. Design strategies to

improve thiophene-based polymeric HTMs have included the addition of acceptor units,

such as benzothiadiazole (e.g., poly-[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-

ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl]] or PCDTBT).69 Although

112

this particular polymer showed an increased the Voc of PSC devices, the Jsc was

significantly lower than analogous devices with P3HT as the HTM. Alternatively,

insertion of a DPP unit (e.g., poly[2,5-bis(2-decyldodecyl)-pyrrolo[3,4-c]pyrrole-

1,4(2H,5H)-dione-(E)-1,2-di(2,20-bithio phen-5-yl)-ethene] or PDPPDBTE in Figure

E2)113 to a thiophene-conjugated backbone effectively reduced the HOMO, producing a

polymeric HTM demonstrating a much higher Voc while maintaining a similar Jsc as

devices with P3HT.

Figure E2. Polymer HTMs for PSCs with thiophene functionalities highlighted in red.

113

APPENDIX F – THE EFFECT OF DOPANTS ON HOLE-TRANSPORT

Although it is well-known that the addition of p-type dopants to spiro-OMeTAD

greatly enhance device performance (see Appendix F for experimental data), 114–116 these

additives are hygroscopic and detrimental to the long-term stability of the cell.110 Figure

F1 depicts the J-V curves of devices fabricated with Spiro-OMeTAD doped with Li-TFSI

and tBP and without dopants. The devices with the doped HTM had a higher FF and higher

Voc as a result of lower series resistance.

Figure F1. A comparison of J-V curves of the best-performing devices with Spiro-

OMeTAD doped with Li-TFSI and tBP against devices undoped Spiro-OMeTAD as the

HTL. The plots were obtained by scanning from +2.0 V to 0.0 V at a rate of 10 ms per data

point under 1.5 G (one Sun) illumination.

114

APPENDIX G – THE EFFECT OF NON-CONJUGATED POLYMER

ADDITIVES ON FILM FORMATION OF HOLE-TRANSPORT LAYERS

Non-conjugated polymers were combined with HTM04 to observe the effect of

inert additives in the HTL of PSCs. The binary blends were made by spin-coating HTL

precursor solutions containing polystyrene (PS) or trimethyl-terminated

polydimethylsiloxane (PDMS) and HTM04 dissolved in CB. The concentration of HTM04

was kept at 25 mg/mL and the amount of polymer was equivalent to 2% of the HTM by

weight (i.e., 0.5 mg/mL). PS was selected because it has been shown to improve the

performance of small-molecule bulk-heterojunction OSCs with HTM04 as the donor.117

PDMS was selected for its previous success as an inert additive in HTLs with another

thiophene-containing small-molecule HTM. 110 It was found that the addition of PDMS

helped planarize the surface of the HTL, resulting in an increase in Jsc and FF.

Indeed, devices with binary blends of HTM04 and PDMS showed higher Jsc than

their neat-film counterparts, contributing to a better PCE (3.33 % versus 2.76 % for the

HTM04 neat film). Devices with HTM04 with 2% PS showed little difference in

performance compared to the neat-film devices (see Figure G1).

Figure G1. A comparison J-V curves for the best-performing devices with HTM04 with

and without non-conjugated polymer additives. The plots were obtained by scanning from

+2.0 V to 0.0 V at a rate of 10 ms per data point under 1.5 G illumination.

115

Table G1. Summary of J-V characteristics of devices with HTM04 as the HTM with and

without non-conjugated polymer additives. All measurements were taken by scanning

from +2.0 V to 0.0 V with a scan delay of 10 ms under 1.5 G illumination.

HTL Voc (V) Jsc (mA

cm-2)

PCE

(%) FF

Shunt

Resistance

(Ω/cm2)

Series

Resistance

(Ω/cm2)

HTM04 neat 0.89 10.02 2.76 0.31 1.74×105 7.46×10-2

HTM04 + 2% PS 0.87 9.35 2.71 0.33 2.13×105 7.34×10-2

HTM04 + 2% PDMS 0.85 11.87 3.33 0.33 2.20×105 7.58×10-2

AFM images of the HTL surfaces reveal that films with HTM04 and PDMS on a

perovskite active layer are less rough than HTM04 neat films on perovskite (see Figure

G2). The images suggest that the addition of PDMS planarizes the HTL by filling in the

deep “valleys” in the perovskite film. On the other hand, the roughness of the films with

and without PS are very similar. Interestingly, addition of either polymer improved the

shunt resistance of the devices, which gives further evidence that the blends are providing

better coverage of the perovskite layer, and thus passivating shunt pathways.

Figure G2. 2D AFM images of HTL surfaces of HTM04 with and without polymer

additives on a perovskite film. An image of a neat perovskite film is shown for comparison.

116

Table G2. Roughness parameters for HTL films on perovskite obtained from 2D AFM

images.

No HTL HTM04

neat

HTM04 +

2% PS

HTM04 + 2%

PDMS

Surface Area (µm²) 26 23 24 25

RRMS (nm) 35 20 17 14

Ra (nm) 28 14 12 11

Rmax (nm) 246 170 139 139 a RRMS, the root mean square (RMS) average of height deviations relative to the mean image data plane. b Ra,

the average of the absolute values of the surface height deviations relative to the mean plane. c Rmax, the

vertical distance between the highest and lowest data points in the image.

In conclusion, it has been shown that addition of PDMS increases planarity of the

HTM04 surface and contributes to higher photocurrent than neat-films of HTM04. This

strategy for device improvement seems to be especially important when fabricating devices

with PHJ perovskite active layers in which large perovskite crystal domains are common

and difficult to cover.

117

APPENDIX H – 3D NON-FULLERENE ACCEPTORS

A well-studied strategy in the design of high-performing non-fullerene acceptors is

mimicry of the advantageous 3D shape of PCBM. Three categories of 3D non-fullerene

acceptors are: i) twisted dimers, ii) tetramers and iii) TPA-based star-shapes. Jenekhe and

co-workers118 have demonstrated a successful design methodology for the synthesis of non-

planar non-fullerene acceptors (see Figure H1 for structure); 3D twisted dimeric structures

were shown to exhibit more homogeneous thin-films than their 2D monomeric units,

ultimately delivering higher performance in OPV devices with a 2D polymer donor. Other

3D dimeric structures include bridged perylene-diimide (PDI) derivatives.119–121

Non-planar tetrameric non-fullerene acceptors contain four conjugated units spatially

distributed around a common core. One strategy is to use a tetrahedral sp3-hybridized C122

or Si123 core with four arms. Recently, Yan et al. fashioned a tetramer with four PDI units

bridged at a tetraphenylethylene core, yielding an impressive PCE of 5.5% (Figure H1).124

The third category is trimeric acceptors that have a TPA core. The first report of a

3D TPA-based acceptor displayed modest performance with a PCE of 1.2% with P3HT as

the polymer donor.125 Two years later, the same group improved upon their original design

by using electron-deficient PDI units and synthesizing a new TPA-based acceptor with

higher electron mobility and better PV performance.126

118

Figure H1. Examples of non-fullerene acceptors, including bridged dimers (a)118 and

b)119), c) a tetramer124, and d) a TPA trimer with PDI side-arm units.126

119

APPENDIX I – VARYING DONOR-ACCEPTOR RATIOS

Figure I1 depicts UV-Vis spectra of D1-A1 and D2-D1 blends spin-cast from

solutions with varying D:A weight ratios. It was confirmed that by reducing the D:A weight

ratio, the relative absorption intensity of A1 increases, balancing the absorption of the film

across the spectrum. However, if the concentration of the precursor solution is fixed at 30

mg/mL, while reducing the D:A ratio, the total absorbance of the film is reduced. Since

there is a trade-off between absorption balance and total absorbance, only fabrication of

devices with the different ratios can determine the optimal ratio for OPVs.

Figure I1. UV-Vis absorption spectra acquired for as-cast blend films of a) D1-A1 and b)

D2-A1 with varying D:A weight ratios. Spectra were acquired by Arthur Hendsbee.

120

Figure I2. XRD patterns of thin-film blends with D1-A1 ratios of a) 1:1 and b) 1:2 acquired

before and after thermal annealing for 120 min at 150 °C. The A1 diffraction peak is higher

in relative intensity in the pattern for the 1:2 film than in the 1:1 due to the higher quantity

of crystalline material.

121

APPENDIX J – SOLVENT ADDITIVES AND SOLVENT VAPOUR ANNEALING

In Chapter 4, thermal annealing was used as a processing technique to modify the

structure and morphology of thin film blends of D1-A1. Two other techniques that can be

applied to formation of thin films and the fabrication of OSCs are the use of solvent

additives and solvent vapour annealing.

Solvent Additives

Solvent additives are typically high-boiling liquids that are co-dissolved with the

photoactive materials in the spin-coating solution. The most common solvent additive is

1,8-diiodooctane (DIO), which has been shown to yield remarkable improvements in a

variety of OPV systems.127–129 Nevertheless, other solvent additives have also been

shown to induce different and favourable changes in thin film OSCs.130

In a preliminary study, three different high-boiling compounds were selected as

solvent additives for the D1-A1 systems. In separate samples, either DIO, 4-

trifluoromethyl benzoic acid (4-TFBA) or 1-naphthaldehyde (1-NA) were used as solvent

additives in the solutions containing D1 and A1 in a 1:1 weight ratio with a total solid

concentration of 20 mg/mL. A stock solution of each solvent additive was first prepared

in CHCl3 at a concentration of 4% (v/v%). D1-A1 solutions were prepared by combining

the weighed solid compounds with a measured mixture of the solvent-additive stock

solution and pure CHCl3 as per the volumes listed in Table J1. All volumes of solvent and

solvent additive were measured with either a 200-μL or 1000-μL micropipette.

Table J1. Compositions of solvents with solvent-additive.

Volume %

Additive

Total Solution

Volume (μL)

Volume of

Additive (μL)

Volume of

Stock Solution

(μL)

Volume of

Pure CHCl3

(μL)

0.4 1000 4 100 900

1.0 1000 10 250 750

122

From the UV-Vis spectra depicted in Figure J1, it is observed that small

quantities of each of the solvent additives have negligible effects on the absorption

properties of D1 and A1. However, increasing the concentration of the DIO (Figure J1

a)) and 1-NA (Figure J1 c)) to 1.0% results in changes to the absorption profile of A1

and emergence of three distinct bands and a reduction in the intensity of the high-energy

absorption band of D1. On the other hand, addition of 1.0% of 1-TFBA results in a

broadening of the A1 absorption, but no significant changes in the absorption of D1.

Although it is clear that electronic changes occur within the films from the presence of

the solvent additives, structural changes could not be detected using XRD (Figure J1 d),

e), f)). In this case, AFM is an ideal tool to probe the films for morphological changes.

However, these experiments have yet to be performed.

Figure J1. UV-Vis spectra of D1-A1 blend thin films (1:1 weight ratio) with various

amounts of solvent additives: a) DIO, b) 4-TFBA and c) 1-NA. Also shown are XRD

patterns of the blend films with d) DIO, e) 4-TFBA and f) 1-NA.

123

Solvent Vapour Annealing

Solvent vapour annealing (SVA) involves the exposure of a thin film to a gaseous

solvent for a given amount of time (see Figure J2).

Figure J2. A photograph of a D1-A1 thin film undergoing SVA in an annealing

chamber.

An SVA chamber was prepared by dispensing 2000 μL of CHCl3 into a clean and

dry shot glass with a lid covering the mouth. The solvent vapour was allowed to saturate

the interior of the chamber for 10 minutes. To anneal a film, the film-coated substrate was

suspended above the solvent before re-attaching the lid. The vapour was allowed to

penetrate the film for a set amount of time before the substrate was removal. The film was

allowed to dry for 2 minutes before a UV-Vis spectrum was acquired. The film was then

returned to the chamber and annealed for additional time. This process of sequential

annealing with intermittent acquisition of UV-Vis spectra was repeated until the total

amount of time spent in the chamber was equal to 20 minutes. An XRD pattern of the film

after SVA for a total time of 20 minutes was also acquired.

124

From the UV-Vis spectra in Figure J3, one can observe changes in the absorption

profile after only a short duration of SVA (i.e., 60 s). The change in the band structure of

A1 as a result of SVA is very similar to that observed after the use of DIO and 1-NA as

solvent additives. No evidence of crystallinity in the films after SVA were detected using

XRD. Again, AFM is an invaluable tool for probing the morphology of this system and

will be applied in future experiments.

Figure J3. a) UV-Vis spectra of a D1-A1 blend thin film (1:1 weight ratio) before and after

sequential SVA. b) XRD patterns of a D1-A1 blend thin film before and after SVA for 20

min.

Documents

A Study of Photoactive Materials for Solution-Processed